in-c

; An Atari 800XL with a double-density disk drive: sixty-four kilobytes and no
; banking, so storage is the diskette's own sectors, which nothing maps into
; memory. There is no unit set and no Window for that reason, and the driver
; shows nothing. The Container is the `.atr`, a bootable diskette whose first
; three sectors the tool writes itself.
target {
  cpu "6502"

  ; A diskette it boots, or a `.xex` a DOS loads. A Project naming any other
  ; is refused here rather than at the writer.
  containers atr, xex
}

; The diskette's sectors, from the fourth: the boot record has the first three
; and the load image whatever is left after the storage a program uses. A unit
; is 256 sectors, which is the one size the `.atr` takes, and three of them
; reach past the 180 KB a double-density diskette holds. It is a ceiling and
; not a reservation: what the image costs is what the program's Payloads came
; to.
storage { units 3 size 65536 }

target {
  region ram    $0000 .. $06FF  ram
  ; $0700 to $087F is the boot record's. It stands in no Region, so the solver
  ; never allocates there and the record's own pins are the author's word.
  region        $0880 .. $CFFF  ram
  region        $D800 .. $FFFF  ram
  region stack  $0100 .. $01FF  reserved
  region io     $D000 .. $D7FF  register

  ; A Project that wants more of the hardware by name declares it, as it
  ; declares the memory its DOS holds.
  register PORTB  $D301
}

; The OS, the driver for a diskette and the decoders the tool ships, listed as
; a file of the Project's own would be. Every Transition calls the driver and
; the decoders, so they are resident, and the OS is resident because the driver
; calls into it; a decoder nothing uses is dropped.
modules { "atari/os.asm"  "atari/charsets.asm"  "atari/disk.asm"  "stream/zx0.asm" }
resident { os, charsets, disk, zx0 }

; The Atari OS as a Module: what it occupies while it is in memory, and the
; names a program reaches it by. Sections that hold no bytes, pinned where
; the OS lives, so the solver sees memory that is taken rather than a Region
; it may never allocate from — which is what lets a Phase that switches the
; ROM out have those addresses back, once there is a way to say so. See
; 0038 for why what occupies memory for a while is a Module, and 0051 for
; what this first cut settles and what it leaves.
;
; Every Section here is `root`: the OS and the hardware reach them with no
; Reference in any Chunk, so nothing else would keep them. None holds bytes,
; so none has a Payload and no Transition loads one.
;
; This is the OS as an XL or XE ships it. A machine whose OS lives elsewhere
; declares a Module of its own; the variant is what chooses.

.export RTCLOK, SDMCTL, SDLSTL, SDLSTH, CH, SETVBV, XITVBV

; The OS's half of the zero page. The program's own variables live above it,
; in the half the variant leaves to the solver.
.section zeropage at $0000, root
osZero
        .res $12
RTCLOK  .res 3                          ; $0012: the frame counter, three bytes
        .res $6B
.ends

; The OS's variables and buffers. The shadow registers are among them and
; are Labels here rather than registers of the variant: they are the OS's
; memory, which the OS copies to the hardware on every vertical blank, and
; a `register` Region inside this Section would be an address the Section
; may not cover.
.section absolute at $0200, root
osRam
        .res $2F
SDMCTL  .res 1                          ; $022F: shadows DMACTL
SDLSTL  .res 1                          ; $0230: the display list address, low
SDLSTH  .res 1                          ; $0231: and high
        .res $CA
CH      .res 1                          ; $02FC: the last key pressed, $FF for none
        .res $403
.ends

; The ROM, in the two ranges the hardware registers leave between them, and
; the entry points a program calls in the second. The RAM underneath is what
; a Phase without the OS would be given.
.section absolute at $C000, root
osRomLow
        .res $1000
.ends

.section absolute at $D800, root
osRomHigh
        .res $C5C
SETVBV  .res 3                          ; $E45C: A = the stage, X/Y = the routine
        .res 3
XITVBV  .res 3                          ; $E462: the end of a deferred routine
        .res $1B9B
.ends

; The Atari's two codes for the same letters, as Charsets.
;
; `atascii` is what the character I/O takes: the machine's own code, with the
; graphics characters where an ASCII machine keeps its control codes, and the
; letters where ASCII has them — which is why an unprefixed literal of plain
; letters has always worked and why nothing else has. `\n` is `$9B`, the end of
; line, and not `$0A`.
;
; `screen` is what the display reads out of screen memory, which is the same
; letters at other numbers. A program that writes where the display looks
; rather than through the OS wants this one.
;
; The graphics characters are written here as the Unicode the box-drawing and
; block characters have, so a picture drawn in the source is the picture the
; machine draws. A letter in inverse video is one of those too — Unicode squares
; them off in a negative, and `atascii"PRESS 🆂"` is seven bytes with the last
; one inverse. A whole line of inverse text is better asked of a Charset
; derived from this one, `.charset bright : atascii ^ $80`, which is what a
; derivation is for. Two of them have no exact Unicode: `$02` and `$0D` are a
; quarter of a cell where the nearest character is an eighth, and are written
; as `▕` and `▔`. Everything else, the inverse entries included, was read off
; the machine's own font and matches it.
;
; This Module emits nothing: it is two names and two tables.

.export atascii, screen

; tag atascii
.charset atascii
  "♥├▕┘┤┐╱╲◢▗◣▝▘▔▂▖♣┌─┼●▄▎┬┴▌└"                                       = $00
  "↑↓←→"                                                              = $1C
  " !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_" = $20
  "♦"                                                                 = $60
  "abcdefghijklmnopqrstuvwxyz"                                        = $61
  "♠"                                                                 = $7B
  "│"                                                                 = $7C
  "\n"                                                                = $9B

  ; Inverse video is the same glyph with the bits the other way round, which
  ; for the blocks and the triangles is another character Unicode draws. Every
  ; one of these was read off the machine's own font and is exact; the rest of
  ; the inverse half has no glyph to be written as, and is reached by adding
  ; $80 or by a Charset derived from this one.
  "▊" = $82        ; ▕
  "◤" = $88        ; ◢
  "▛" = $89        ; ▗
  "◥" = $8A        ; ◣
  "▙" = $8B        ; ▝
  "▟" = $8C        ; ▘
  "▆" = $8D        ; ▔
  "▜" = $8F        ; ▖
  "▀" = $95        ; ▄
  "▐" = $99        ; ▌
  "█" = $A0        ; the space

  ; And the letters, which Unicode squares off in a negative.
  "🅰🅱🅲🅳🅴🅵🅶🅷🅸🅹🅺🅻🅼🅽🅾🅿🆀🆁🆂🆃🆄🆅🆆🆇🆈🆉" = $C1
.endch
; end atascii

.charset screen
  " !\"#$%&'()*+,-./0123456789:;<=>?"                                 = $00
  "@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_"                                 = $20
  "♥├▕┘┤┐╱╲◢▗◣▝▘▔▂▖♣┌─┼●▄▎┬┴▌└"                                       = $40
  "♦"                                                                 = $60
  "abcdefghijklmnopqrstuvwxyz"                                        = $61
  "♠"                                                                 = $7B
  "│"                                                                 = $7C

  ; The same inverses, at the display's numbers.
  "▊" = $C2
  "◤" = $C8
  "▛" = $C9
  "◥" = $CA
  "▙" = $CB
  "▟" = $CC
  "▆" = $CD
  "▜" = $CF
  "▀" = $D5
  "▐" = $D9
  "█" = $80
  "🅰🅱🅲🅳🅴🅵🅶🅷🅸🅹🅺🅻🅼🅽🅾🅿🆀🆁🆂🆃🆄🆅🆆🆇🆈🆉" = $A1
.endch

; The storage driver for a diskette: a unit of storage is 256 sectors of 256
; bytes, and unit zero begins at sector 4, the first after the boot record.
; That size is what makes the three bytes the model addresses storage with —
; a unit and a two-byte offset — the address the hardware takes, so this driver
; does no arithmetic: the unit is the sector number's high byte, the offset's
; high byte is its low one, and the offset's low byte is the position in the
; sector. Reading a sector is the OS's own disk handler, called through
; `DSKINV` with the request in the device control block, which is why this
; driver needs no register of its own and knows nothing about a drive.
;
; The medium maps nothing, so this driver names no Window: `stream`, `show`
; and `showAt` are not declared, and the two Procs that put base memory back
; after a Transition are a `rts`.
;
; Resident, since every Transition calls it; the variant lists it so.

DSKINV = $E453                  ; the OS's disk handler: one sector per call
DUNIT  = $0301                  ; the drive, 1 for D1:
DCOMND = $0302                  ; 'R' reads
DBUFLO = $0304
DBUFHI = $0305
DAUX1  = $030A                  ; the sector, two bytes
DAUX2  = $030B

diskFirst = 4                   ; the sector unit zero begins at

.driver open diskOpen
.driver read diskRead

.transform copy diskCopy

.macro diskOpen
        jsr diskOpenStream
.endm

.macro diskRead
        jsr diskReadByte
.endm

; The stream: which sector the buffer holds, where in it the next byte is, and
; whether the position has run off its end. A plain Section rather than
; Temporaries, because the value has to survive between one call and the next
; while nothing here is running.
.section zeropage
diskAt          .res 2
diskPos         .res 1
diskNeed        .res 1
.ends

; The sector the stream reads through. A Section of reservations alone, so it
; is in no Container and no load writes it. `within 256` keeps it off a page
; boundary: every byte of it is read as `diskBuffer,y`, and an index that
; carries into the next page costs a cycle.
.section absolute within 256
diskBuffer
        .res 256
.ends

; A = the unit, X/Y = the offset in it: the stream stands there. An offset past
; the unit's end carries into the units after, since the routine counts on in a
; Frame without knowing where a unit ends — and here it carries by itself, the
; three bytes being one position that the sector number takes the top two of.
.proc diskOpenStream
        stx diskPos
        sta diskAt+1            ; the unit is the sector number's high byte
        tya                     ; and the offset's high byte is its low one
        clc
        adc #<diskFirst
        sta diskAt
        lda diskAt+1
        adc #>diskFirst
        sta diskAt+1
        jmp diskFill
.endp

; A = the next byte of the stream. X and Y are not preserved.
.proc diskReadByte
        lda diskNeed
        beq @have
        inc diskAt
        bne @next
        inc diskAt+1
@next
        jsr diskFill
@have
        ldy diskPos
        inc diskPos
        bne @within
        sty diskNeed            ; Y is $FF here, and any non-zero will do
@within
        lda diskBuffer,y
        rts
.endp

; The sector in diskAt into the buffer, asked for again until the handler says
; it has it: a Transition that cannot read cannot go on, and the drive answered
; when the OS read the boot record off this diskette.
.proc diskFill
        lda #0
        sta diskNeed
        lda diskAt
        sta DAUX1
        lda diskAt+1
        sta DAUX2
@again
        lda #1
        sta DUNIT
        lda #$52
        sta DCOMND
        lda #<diskBuffer
        sta DBUFLO
        lda #>diskBuffer
        sta DBUFHI
        jsr DSKINV
        bmi @again
        rts
.endp

; The decoder of `copy`: X/Y = the destination, the stream at the stored size
; and then the bytes. A byte at a time through the stream, because what this
; costs is the sector reads underneath it — a run copied out of the buffer
; would save a few hundred cycles against the tens of thousands a sector takes
; to arrive.
diskDst  .ztemp 2
diskLeft .ztemp 2

.proc diskCopy
        stx diskDst
        sty diskDst+1
        jsr diskReadByte
        sta diskLeft
        jsr diskReadByte
        sta diskLeft+1
@byte
        lda diskLeft
        ora diskLeft+1
        beq @done
        jsr diskReadByte
        ldy #0
        sta (diskDst),y
        inc diskDst
        bne @counted
        inc diskDst+1
@counted
        lda diskLeft
        bne @low
        dec diskLeft+1
@low
        dec diskLeft
        jmp @byte
@done
        rts
.endp

; The decoder of `zx0` over any driver's stream: Einar Saukas's ZX0, version
; 2's standard forward stream, decoded as the reference dzx0.c does, driven
; by the stream's own end marker. Written here from the format, not ported:
; nothing of the reference's text is in it, and it is licensed as everything
; under lib/ is, see lib/LICENSE. Entered with X/Y = the destination and the
; stream at the stored size, which it reads past. A match is copied a byte
; at a time, forward, from what was written, which is what makes an offset
; shorter than its length — the run — come out right. Resident, and outside
; the driver's window.
;
;   zx0Bits      the bit buffer, a sentinel one above the bits still unread
;   zx0Offset    the last offset
;   zx0Length    the length in hand, or an offset's MSB while one is read
;   zx0Invert    one while an offset's MSB is read, whose data bits the
;                stream carries complemented; zero otherwise

.transform zx0 zx0Decode

zx0Dst    .ztemp 2
zx0Src    .ztemp 2
zx0Bits   .ztemp 1
zx0Offset .ztemp 2
zx0Length .ztemp 2
zx0Invert .ztemp 1

; The next bit of the stream, in A as zero or one and in the Z flag. The
; buffer holds a sentinel above the unread bits, so shifting it to nothing is
; the signal to fetch the next byte and put the sentinel back below it.
.proc zx0Bit
        asl zx0Bits
        bne @have
        nga.read
        rol                     ; the carry the asl left is the sentinel
        sta zx0Bits
@have
        lda #0
        rol
        rts
.endp

; An interlaced Elias gamma value into zx0Length: a control bit says whether
; a data bit follows, and the value begins at one. Three Procs chained by
; `then` because the value after an offset is entered with its first control
; bit already read, at zx0EliasData: the first falls through into the
; second, and the second branches into the third, which the chain is what
; allows.
.proc zx0Elias
        lda #1
        sta zx0Length
        lda #0
        sta zx0Length+1
.endp then zx0EliasMore
.proc zx0EliasMore
        jsr zx0Bit
        beq zx0EliasData        ; zero: a data bit follows
        rts
.endp then zx0EliasData
.proc zx0EliasData
        jsr zx0Bit
        eor zx0Invert
        lsr
        rol zx0Length
        rol zx0Length+1
        jmp zx0EliasMore
.endp

; One byte written: the destination moves on and the length in hand comes
; down, leaving Z set when it reaches zero.
.proc zx0Step
        inc zx0Dst
        bne @moved
        inc zx0Dst+1
@moved
        lda zx0Length
        bne @low
        dec zx0Length+1
@low
        dec zx0Length
        lda zx0Length
        ora zx0Length+1
        rts
.endp

; The length in hand copied from zx0Dst less the last offset to zx0Dst.
.proc zx0Copy
        sec
        lda zx0Dst
        sbc zx0Offset
        sta zx0Src
        lda zx0Dst+1
        sbc zx0Offset+1
        sta zx0Src+1
@byte
        ldy #0
        lda (zx0Src),y
        sta (zx0Dst),y
        inc zx0Src
        bne @from
        inc zx0Src+1
@from
        jsr zx0Step
        bne @byte
        rts
.endp

.proc zx0Decode
        stx zx0Dst
        sty zx0Dst+1
        nga.read             ; the stored size, which the end marker makes unnecessary
        nga.read
        lda #$80
        sta zx0Bits             ; an empty buffer: the sentinel alone
        lda #1
        sta zx0Offset
        lda #0
        sta zx0Offset+1         ; the last offset begins at one
        sta zx0Invert
@literals
        jsr zx0Elias
@literal
        nga.read
        ldy #0
        sta (zx0Dst),y
        jsr zx0Step
        bne @literal
        jsr zx0Bit
        bne @offset
        jsr zx0Elias            ; a match at the last offset
        jsr zx0Copy
        jsr zx0Bit
        beq @literals
@offset
        lda #1
        sta zx0Invert
        jsr zx0Elias            ; the new offset's MSB, complemented in the stream
        lda #0
        sta zx0Invert
        lda zx0Length
        beq @done               ; 256 is the end marker, and the one value with a low byte of zero
        lsr                     ; offset = MSB * 128 - LSB / 2
        sta zx0Offset+1
        lda #0
        ror
        sta zx0Offset
        nga.read
        lsr                     ; the LSB's low bit is the length's first control bit
        php
        eor #$FF
        sec
        adc zx0Offset
        sta zx0Offset
        bcs @subtracted
        dec zx0Offset+1
@subtracted
        plp
        lda #1
        sta zx0Length
        lda #0
        sta zx0Length+1
        bcs @counted            ; a control bit of one: the value is one
        jsr zx0EliasData
@counted
        inc zx0Length           ; a match at a new offset is one longer than written
        bne @copy
        inc zx0Length+1
@copy
        jsr zx0Copy
        jsr zx0Bit
        bne @offset
        jmp @literals
@done
        rts
.endp

.source "intro.ngc", 1
.section readonly
introText
        .byte 73, 78, 84, 82, 79, 0
.ends

.source "intro.ngc", 3
.export introStart
.proc introStart
        .source "intro.ngc", 5
        ldx #<introText
        ldy #>introText
        lda #5
        jsr printLine
        .source "intro.ngc", 6
        .transition level
.endp

.source "level.ngc", 1
.section readonly
levelMap
        .byte 35, 35, 35, 35, 35, 35, 35, 35
        .byte 35, 35, 35, 35, 35, 35, 35, 35
        .byte 35, 35, 35, 35, 35, 35, 35, 35
        .byte 35, 35, 35, 35, 35, 35, 35, 35
        .byte 35, 35, 35, 35, 35, 35, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 35, 35, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 35, 35
        .byte 46, 46, 46, 46, 46, 46, 35, 35
        .byte 35, 35, 35, 35, 35, 35, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 35, 35, 46, 46, 46
        .byte 46, 46, 46, 35, 46, 46, 46, 46
        .byte 46, 46, 35, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 35, 35, 46, 46, 46, 46, 46, 46
        .byte 35, 46, 46, 46, 46, 46, 46, 35
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 35, 35, 46
        .byte 46, 46, 46, 46, 46, 35, 35, 35
        .byte 35, 35, 35, 35, 35, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 35, 35, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 35
        .byte 35, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 46, 46, 46, 46
        .byte 46, 46, 46, 46, 35, 35, 35, 35
        .byte 35, 35, 35, 35, 35, 35, 35, 35
        .byte 35, 35, 35, 35, 35, 35, 35, 35
        .byte 35, 35, 35, 35, 35, 35, 35, 35
        .byte 35, 35, 35, 35, 35, 35, 35, 35
        .byte 35, 35, 0
.ends

.source "level.ngc", 13
.export levelStart
.proc levelStart
        .source "level.ngc", 15
        lda #<levelMap
        sta __0at
        lda #>levelMap
        sta __0at+1
        .source "level.ngc", 16
        lda #10
        sta __1left
@l1
        lda __1left
        jeq @l5
        .source "level.ngc", 18
        ldx __0at
        ldy __0at+1
        lda #37
        jsr printLine
        .source "level.ngc", 19
        clc
        lda __0at
        adc #37
        sta __0at
        lda __0at+1
        adc #0
        sta __0at+1
        .source "level.ngc", 16
        dec __1left
        jmp @l1
@l4
@l5
        .source "level.ngc", 21
        jmp @l4
__0at .ztemp 2
__1left .ztemp 1
.endp

; The Transition routine. Called by `jsr` from a `.transition`, with the
; statement's list right behind the call: the Phase to enter, and per Phase
; the code may be in, the unit and offset where the edge's Frame waits.
; nga.open and nga.read are the driver's roles, macros of the driver's
; Module expanded here; ngaShowBases is the Proc the tool generates beside
; the dispatcher, which reads from the Frame the base the entered Phase gives
; every Window and shows it, the stream's last; and ngaCurrentPhase is the
; Cell the tool generates. Everything else the edge has to say waits in the
; Frame, in storage. See docs/spec/transition.md.
;
; Its zero page is Temporaries: nothing of it is needed once the routine has
; jumped to the entered Phase's entry, and a `.transition` is a jump into the
; routine, so whatever the statement's Section had on the zero page is dead by
; then too. Two Temporaries never live at once share an address — see
; docs/decisions/0034-trace.md.

.export ngaTransition

ngaPtr .ztemp 2
ngaEntry .ztemp 2
ngaDst .ztemp 2
ngaValue .ztemp 2
ngaOffset .ztemp 2
ngaFramePos .ztemp 2
ngaWanted .ztemp 1
ngaCount .ztemp 1
ngaFrameUnit .ztemp 1
ngaUnit .ztemp 1

.proc ngaTransition
        pla
        sta ngaPtr
        pla
        sta ngaPtr+1            ; the return address: one below the statement's list
        ldy #1
        lda (ngaPtr),y
        sta ngaWanted           ; the Phase to enter
        iny
        lda (ngaPtr),y
        sta ngaCount            ; entries that follow
        lda ngaPtr
        clc
        adc #3
        sta ngaPtr
        bcc @scan
        inc ngaPtr+1
@scan
        lda ngaCount
        bne @check
        brk                     ; no entry for the current Phase: unreachable while the static rule holds
@check
        ldy #0
        lda (ngaPtr),y
        cmp ngaCurrentPhase
        beq @found
        lda ngaPtr              ; the next entry: four bytes on
        clc
        adc #4
        sta ngaPtr
        bcc @skipped
        inc ngaPtr+1
@skipped
        dec ngaCount
        jmp @scan
@found
        iny
        lda (ngaPtr),y
        sta ngaFrameUnit        ; the unit the Frame waits in
        iny
        lda (ngaPtr),y
        sta ngaFramePos
        iny
        lda (ngaPtr),y
        sta ngaFramePos+1       ; and where in it
.endp then ngaEnter

; The rest of it, and the way in for a Container that has no current Phase to
; look one up by: ngaWanted holds the Phase to enter and ngaFrameUnit and
; ngaFramePos where its Frame waits. The cold start comes here — see
; docs/decisions/0216-a-car-names-its-format-and-the-cold-start-is-an-edge.md.
.export ngaEnter
.proc ngaEnter
        jsr ngaFrameOpen
        nga.read
        sta ngaEntry
        nga.read
        sta ngaEntry+1
        nga.read
        sta ngaCount            ; blocks that follow
        lda #3
        jsr ngaFrameSkip
@block
        lda ngaCount
        beq @cells
        jsr ngaFrameOpen        ; back to the Frame: a block's stream replaced it
        nga.read
        sta ngaUnit
        nga.read
        sta ngaOffset
        nga.read
        sta ngaOffset+1
        nga.read
        sta ngaDst
        nga.read
        sta ngaDst+1
        nga.read
        pha                     ; the decoder's number
        lda #6
        jsr ngaFrameSkip
        lda ngaUnit
        ldx ngaOffset
        ldy ngaOffset+1
        nga.open                ; the block's stream: its stored size, then its bytes
        pla
        ldx ngaDst
        ldy ngaDst+1
        jsr ngaTransform
        dec ngaCount
        jmp @block
@cells
        jsr ngaFrameOpen
        nga.read
        sta ngaCount            ; Cell writes that follow, read in one stream
@cell
        lda ngaCount
        beq @enter
        nga.read
        sta ngaDst
        nga.read
        sta ngaDst+1
        nga.read
        sta ngaValue
        nga.read
        sta ngaValue+1
        ldy #0
        lda ngaValue
        sta (ngaDst),y          ; a Cell stands outside the driver's window
        iny
        lda ngaValue+1
        sta (ngaDst),y
        dec ngaCount
        jmp @cell
@enter
        jsr ngaShowBases        ; the entered Phase's base in every Window, read from the Frame
        lda ngaWanted
        sta ngaCurrentPhase     ; the Phase the program is in from here on
        jmp (ngaEntry)
.endp

; The Frame's stream, from where the routine last left it.
.proc ngaFrameOpen
        lda ngaFrameUnit
        ldx ngaFramePos
        ldy ngaFramePos+1
        nga.open
        rts
.endp

; A bytes on in the Frame, which is where its stream is opened next.
.proc ngaFrameSkip
        clc
        adc ngaFramePos
        sta ngaFramePos
        bcc @done
        inc ngaFramePos+1
@done
        rts
.endp

ngaCurrentPhase

; The boot record of an .atr: the first three sectors, which the OS reads to
; $0700 and calls at $0706. Those three hold 128 bytes, because the OS boots
; with DSCTLN at 128; every sector after them holds 256, so the first thing the
; loader does is say so. What it loads is the load image, the stream of segments
; a `.xex` is made of with $FFFF closing it, waiting in the sectors after
; storage. See docs/spec/atr.md.

DSKINV = $E453
DUNIT  = $0301
DCOMND = $0302
DBUFLO = $0304
DBUFHI = $0305
DAUX1  = $030A
DAUX2  = $030B
DSCTLN = $02D5                  ; what a sector holds, which DSKINV transfers
RUNAD  = $02E0
DOSVEC = $000A

bootBuf = $0400                 ; the cassette buffer and the spare page after
                                ; it, both free once the record is in: 256
                                ; bytes, which is what a sector holds here

; The zero page the OS leaves once it has booted: RAMLO and CASINI were the
; boot's own scratch and TSTDAT is spare. TRAMSZ, $06, is not among them —
; coldstart reads it after the boot has returned.
bootDst    = $0000              ; where the segment being read lands
bootLeft   = $0002              ; bytes of it still to come
bootSector = $0004              ; the sector the buffer holds
bootPos    = $0007              ; where in the buffer the stream stands, a byte
                                ; that wraps because a sector holds 256

.section absolute at $0700, root
ngaBootRecord
        .byte 0                 ; the OS ignores this byte
        .byte 3                 ; sectors it reads, from the first
        .word $0700             ; where it puts them
        .root                   ; the OS calls what stands here
        .word ngaBootIdle       ; DOSINI, called once the load returns and on reset
.ends

; The first sector of the load image, which the tool writes here once it knows
; how much storage the program came to use, and one byte of the loader's own.
; At the end of the record, out of the loader's way; the zero page the OS
; leaves is spoken for, and $0006 is TRAMSZ, which the cold start reads after
; the boot has returned.
.export ngaBootImage
.section absolute at $087D, root
ngaBootImage
        .word 0
bootNeed
        .res 1                  ; set once the position has wrapped: the next
.ends                           ; byte stands in the sector after

; The OS jsr's here once the three sectors stand at $0700, and a carry set on
; return is a boot it tries again.
.proc ngaBootLoad, absolute at $0706, root
        lda #0                  ; every sector from here on holds 256 bytes
        sta DSCTLN
        lda #1
        sta DSCTLN+1
        lda ngaBootImage
        sta bootSector
        lda ngaBootImage+1
        sta bootSector+1
        jsr fill
@segment
        jsr byte
        sta bootDst
        jsr byte
        sta bootDst+1
        and bootDst             ; $FFFF closes the stream, and no segment starts there
        cmp #$FF
        beq @run
        jsr byte                ; the end address, inclusive
        sta bootLeft
        jsr byte
        sta bootLeft+1
        sec                     ; the count is end - start + 1
        lda bootLeft
        sbc bootDst
        sta bootLeft
        lda bootLeft+1
        sbc bootDst+1
        sta bootLeft+1
        inc bootLeft
        bne @bytes
        inc bootLeft+1
@bytes
        lda bootLeft
        ora bootLeft+1
        beq @segment
        jsr byte
        ldy #0
        sta (bootDst),y
        inc bootDst
        bne @counted
        inc bootDst+1
@counted
        lda bootLeft
        bne @low
        dec bootLeft+1
@low
        dec bootLeft
        jmp @bytes
@run
        lda RUNAD               ; where the program starts, which the OS jumps to
        sta DOSVEC
        lda RUNAD+1
        sta DOSVEC+1
        clc                     ; a boot the OS keeps
        rts

; A = the next byte of the image, which runs from one sector into the next.
; The position wraps to zero at the sector's end, and bootNeed says the buffer
; no longer holds what stands there.
byte
        lda bootNeed
        beq @have
        inc bootSector
        bne @next
        inc bootSector+1
@next
        jsr fill
@have
        ldy bootPos
        inc bootPos
        bne @within
        sty bootNeed            ; Y is $FF here, and any non-zero will do
@within
        lda bootBuf,y
        rts

; The sector in bootSector into the buffer, tried again until it is read: the
; OS read this record off this diskette, so the drive is there.
fill
        lda #0
        sta bootPos
        sta bootNeed
        lda bootSector
        sta DAUX1
        lda bootSector+1
        sta DAUX2
@again
        lda #1
        sta DUNIT
        lda #$52
        sta DCOMND
        lda #<bootBuf
        sta DBUFLO
        lda #>bootBuf
        sta DBUFHI
        jsr DSKINV
        bmi @again
        rts
.endp

; DOSINI: the OS calls it once the load has returned and on every reset, so it
; is in memory by then and the solver places it as it places any Proc.
.proc ngaBootIdle, root
        rts
.endp

__small = 0
; The C runtime: what `*`, `/` and `%` call where no constant makes them
; shifts, adds and masks. Each Proc is called as any Proc of the assembler is:
; its `.declare arg`s written, `jsr`, its `.declare ret` read. A product is the
; low half, one for either sign; a quotient and a remainder truncate towards
; zero, as C does. See docs/decisions/0095-literals-and-the-runtime.md.

.export __mul8, __udiv8, __umod8, __sdiv8, __smod8
.export __mul16, __udiv16, __umod16, __sdiv16, __smod16
.export __mul8to16

; ---------------------------------------------------------------------------
; 8 bits

; left * right: the bits of `left` from the top, the product doubled before
; each and `right` added where the bit is set. The product is left in `A`,
; where it was made — see docs/decisions/0146-the-runtime-in-registers.md.
.proc __mul8, absolute within 256
        .declare arg u8
left    .ztemp 1
        .declare arg u8
right   .ztemp 1
        .declare ret a

        lda #0
        ldx #8
loop    asl                     ; product * 2
        asl left                ; the next bit of `left` into the carry
        bcc skip
        clc
        adc right
skip    dex
        bne loop
        rts
.endp

; left * right, both bytes, the whole product in two: the bits of `left` from
; the bottom, `right` doubled after each and added where the bit is set, the
; product kept in `Y` below and `X` above. Done once no bit of `left` is left,
; so a `left` below 16 takes at most four rounds where every one took eight:
; the compiler hands the operand the ranges hold smaller as `left`. `right` is
; doubled at most seven times, so its pair never carries out of the top — see
; docs/decisions/0144-a-multiply-stops-when-its-multiplier-does.md, and
; docs/decisions/0122-a-multiply-of-two-bytes.md for why there is a Proc of
; two bytes at all. `left` comes in `A`, which is where it is walked — see
; docs/decisions/0146-the-runtime-in-registers.md.
;
; Shaped as Oscar64's `mul16by8` is, which the benchmarks measured it against
; — see docs/decisions/0154-a-multiply-shaped-by-its-bits.md. `A` always holds
; a set bit at `next`, so a clear bit needs no test for zero; the first bit,
; where set, makes the product `right` rather than adding it to zero; and the
; last set bit adds straight into `product`.
.proc __mul8to16, absolute within 256
        .declare arg a
        .declare arg u8
right   .ztemp 1
        .declare ret u16
product .ztemp 2
high    .ztemp 1                ; `right`'s high byte as it is doubled
left    .ztemp 1                ; what is left of `left`, while `A` adds

        ldx #0
        lsr                     ; bit 0 of `left` into the carry
        beq last                ; no bit above it: the product is `right` or 0
        stx high
        ldy #0
        bcc next
        ldy right               ; bit 0 set: the product so far is `right`
        bcs next                ; always
add     sta left                ; what is left of it, while `A` adds
        clc
        tya
        adc right
        tay
        txa
        adc high
        tax
        lda left
next    asl right
        rol high
        lsr                     ; the next bit into the carry
        bcc next                ; clear: `A` still holds a set bit
        bne add                 ; set, and more above it
        clc                     ; set, and the last: added into the product
        tya
        adc right
        sta product
        txa
        adc high
        sta product+1
        rts
last    bcc zero                ; `A` and `X` are zero
        lda right
zero    sta product
        stx product+1
        rts
.endp

; dividend / divisor: the dividend shifted into the remainder in `A` a bit at a
; time, and the divisor subtracted where it fits, which sets the bit of the
; quotient the shift left clear in the dividend's byte. A carry out of the
; shift is a remainder of nine bits, which always fits. A divisor of zero
; always fits: the quotient is all ones.
;
; The quotient is built where the dividend was, a bit a turn, so that byte is
; both the argument and the result and its name says so — see
; docs/decisions/0119-one-temporary-carries-two-roles.md.
.proc __udiv8, absolute within 256
        .declare arg u8
        .declare ret u8
dividendAndQuotient .ztemp 1
        .declare arg u8
divisor .ztemp 1

        lda #0
        ldx #8
loop    asl dividendAndQuotient
        rol                     ; the remainder takes the dividend's top bit
        bcs subtract
        cmp divisor
        bcc skip
subtract
        sbc divisor             ; the carry is set on both ways here
        inc dividendAndQuotient ; the quotient's bit
skip    dex
        bne loop
        rts
.endp

; dividend % divisor: as `__udiv8`, the remainder kept. A divisor of zero leaves
; the dividend.
;
; The remainder is left in `A`, where it was made — see
; docs/decisions/0146-the-runtime-in-registers.md.
.proc __umod8, absolute within 256
        .declare arg u8
dividend .ztemp 1
        .declare arg u8
divisor .ztemp 1
        .declare ret a

        lda #0
        ldx #8
loop    asl dividend
        rol
        bcs subtract
        cmp divisor
        bcc skip
subtract
        sbc divisor
        inc dividend
skip    dex
        bne loop
        rts
.endp

; dividend / divisor, signed: the unsigned quotient of the magnitudes, negated
; where the signs differ.
;
; The dividend comes in `A` and the quotient goes back in it — see
; docs/decisions/0146-the-runtime-in-registers.md.
.proc __sdiv8, absolute within 256
        .declare ret a
        .declare arg a
        .declare arg i8
divisor .ztemp 1
sign    .ztemp 1

        tax                     ; the dividend, while the signs are compared
        eor divisor
        sta sign                ; bit 7: the signs differ

        txa
        bpl dividendKept
        eor #$FF
        clc
        adc #1
dividendKept
        sta __udiv8.dividendAndQuotient

        lda divisor
        bpl divisorKept
        eor #$FF
        clc
        adc #1
divisorKept
        sta __udiv8.divisor

        jsr __udiv8
        lda __udiv8.dividendAndQuotient
        bit sign
        bpl done
        eor #$FF
        clc
        adc #1
done    rts
.endp

; dividend % divisor, signed: the unsigned remainder of the magnitudes, negated
; where the dividend is negative.
;
; The dividend comes in `A` and the remainder goes back in it, as it comes
; back from `__umod8` — see docs/decisions/0146-the-runtime-in-registers.md.
.proc __smod8, absolute within 256
        .declare ret a
        .declare arg a
        .declare arg i8
divisor .ztemp 1
sign    .ztemp 1

        sta sign                ; bit 7: the dividend is negative
        tax                     ; `N` from the dividend: the caller promises none
        bpl dividendKept
        eor #$FF
        clc
        adc #1
dividendKept
        sta __umod8.dividend

        lda divisor
        bpl divisorKept
        eor #$FF
        clc
        adc #1
divisorKept
        sta __umod8.divisor

        jsr __umod8             ; the remainder in `A`
        bit sign
        bpl done
        eor #$FF
        clc
        adc #1
done    rts
.endp

; ---------------------------------------------------------------------------
; 16 bits

; left * right, the low half: the bits of `left` from the bottom, `right`
; doubled after each and added where the bit is set, the product kept in `Y`
; below and `high` above. Done once no bit of `left` is left, which is
; docs/decisions/0144-a-multiply-stops-when-its-multiplier-does.md carried from
; `__mul8to16` to the wider Proc: a multiplier whose high byte is zero takes at
; most eight rounds where every one took sixteen, and one of four bits takes
; four. The round is cheaper as well, `right` being doubled where
; the product was.
;
; `A` walks what is left of the multiplier. Over the low byte a set bit is
; rolled in above it first, so that `A` running out says the byte is done and
; not that the bits above are clear; over the high byte, which is walked at
; `narrow` and is where a multiplier of one byte starts, `A` running out ends
; the Proc. See docs/decisions/0170-a-wide-multiply-stops-when-its-multiplier-does.md.
.proc __mul16, absolute within 256
        .declare arg u16
left    .ztemp 2
        .declare arg u16
right   .ztemp 2
        .declare ret u16
product .ztemp 2
high    .ztemp 1                ; the product's high byte, while `Y` holds its low

.if __small
        ; Sixteen rounds, always, the multiplier walked from the bottom out of
        ; its own bytes and the product added up in memory: half the bytes of
        ; the shape above and none of what makes that one quick.
        lda #0
        sta product
        sta product+1
        ldx #16
@loop
        lsr left+1
        ror left
        bcc @+skip
        clc
        lda product
        adc right
        sta product
        lda product+1
        adc right+1
        sta product+1
@skip
        asl right
        rol right+1
        dex
        bne @-loop
        rts
.else
        ldy #0
        sty high
        lda left
        ldx left+1
        beq @+narrow            ; one byte of multiplier: `A` holds all of it
        sec
        ror                     ; bit 0 out, and a set bit in above the rest
        bcc @+lowNext
@lowAdd
        tax                     ; what is left of the byte, while `A` adds
        clc
        tya
        adc right
        tay
        lda high
        adc right+1
        sta high
        txa
@lowNext
        asl right
        rol right+1
        lsr                     ; the next bit into the carry
        bcc @-lowNext           ; clear: `A` still holds a set bit
        bne @-lowAdd            ; set, and more above it
        lda left+1              ; set, and the mark: the low byte is done
@narrow
        lsr
        bcc @+next
@add
        tax
        clc
        tya
        adc right
        tay
        lda high
        adc right+1
        sta high
        txa
@next
        asl right
        rol right+1
        lsr
        bcs @-add
        bne @-next
        sty product
        lda high
        sta product+1
        rts
.endif
.endp

; dividend / divisor, as `__udiv8` with the remainder in `rest`: the quotient
; is built where the dividend was, so that byte is both the argument and the
; result — see docs/decisions/0119-one-temporary-carries-two-roles.md.
;
; A divisor of one byte below $80 takes the loop at `narrow`, where the
; remainder lives in `A` alone: it stays below the divisor, so doubling it and
; taking a bit of the dividend keeps it inside a byte, and the compare is one
; instruction rather than four. The dividend's own pair carries the quotient
; in at the bottom as it carries the next bit out at the top, so the two
; rotations serve both and nothing counts the bit separately. Half the cycles
; of the wide loop, and it is the loop every call of `prime` takes — see
; docs/decisions/0171-a-divisor-of-one-byte.md. A divisor of zero
; goes wide, which is where the quotient of all ones and the remainder of the
; dividend that docs/spec/c-subset.md states come from.
.proc __udiv16, absolute within 256
        .declare arg u16
        .declare ret u16
dividendAndQuotient .ztemp 2
        .declare arg u16
divisor .ztemp 2
rest    .ztemp 2

.if !__small
        lda divisor+1
        bne wide
        lda divisor
        beq wide
        bmi wide                ; $80 and above: a remainder of nine bits
        ldx #16
        asl dividendAndQuotient
        rol dividendAndQuotient+1
        lda #0
@narrow
        rol                     ; the remainder takes the dividend's top bit
        cmp divisor
        bcc @+narrowSkip
        sbc divisor             ; the carry is set on both ways here
@narrowSkip
        rol dividendAndQuotient ; the quotient's bit in, the next dividend bit out
        rol dividendAndQuotient+1
        dex
        bne @-narrow
        rts
.endif

wide    lda #0
        sta rest
        sta rest+1
        ldx #16
loop    asl dividendAndQuotient
        rol dividendAndQuotient+1
        rol rest                ; the remainder takes the dividend's top bit
        rol rest+1
        bcs subtract
        lda rest                ; rest >= divisor?
        cmp divisor
        lda rest+1
        sbc divisor+1
        bcc skip
subtract
        lda rest
        sec
        sbc divisor
        sta rest
        lda rest+1
        sbc divisor+1
        sta rest+1
        inc dividendAndQuotient ; the quotient's bit
skip    dex
        bne loop
        rts
.endp

; dividend % divisor, as `__udiv16`, the remainder kept: it is built in the
; result's own bytes, and the dividend's, which hold the quotient nothing
; here wants, are the Proc's own. The narrow loop is `__udiv16`'s, and leaves
; in `A` the remainder that Proc throws away — see
; docs/decisions/0171-a-divisor-of-one-byte.md.
.proc __umod16, absolute within 256
        .declare arg u16
dividend .ztemp 2
        .declare arg u16
divisor .ztemp 2
        .declare ret u16
remainder .ztemp 2

.if !__small
        lda divisor+1
        bne wide
        lda divisor
        beq wide
        bmi wide                ; $80 and above: a remainder of nine bits
        lda #0
        sta remainder+1         ; a remainder below the divisor is one byte
        ldx #16
        asl dividend
        rol dividend+1
@narrow
        rol
        cmp divisor
        bcc @+narrowSkip
        sbc divisor
@narrowSkip
        rol dividend
        rol dividend+1
        dex
        bne @-narrow
        sta remainder
        rts
.endif

wide    lda #0
        sta remainder
        sta remainder+1
        ldx #16
loop    asl dividend
        rol dividend+1
        rol remainder
        rol remainder+1
        bcs subtract
        lda remainder
        cmp divisor
        lda remainder+1
        sbc divisor+1
        bcc skip
subtract
        lda remainder
        sec
        sbc divisor
        sta remainder
        lda remainder+1
        sbc divisor+1
        sta remainder+1
        inc dividend
skip    dex
        bne loop
        rts
.endp

; dividend / divisor, signed, as `__sdiv8`.
.proc __sdiv16, absolute within 256
        .declare arg i16
dividend .ztemp 2
        .declare arg i16
divisor .ztemp 2
        .declare ret i16
quotient .ztemp 2
sign    .ztemp 1

        lda dividend+1
        eor divisor+1
        sta sign                ; bit 7: the signs differ

        lda dividend+1
        bmi dividendNegative
        lda dividend
        sta __udiv16.dividendAndQuotient
        lda dividend+1
        sta __udiv16.dividendAndQuotient+1
        jmp dividendDone
dividendNegative
        lda dividend
        eor #$FF
        clc
        adc #1
        sta __udiv16.dividendAndQuotient
        lda dividend+1
        eor #$FF
        adc #0
        sta __udiv16.dividendAndQuotient+1
dividendDone

        lda divisor+1
        bmi divisorNegative
        lda divisor
        sta __udiv16.divisor
        lda divisor+1
        sta __udiv16.divisor+1
        jmp divisorDone
divisorNegative
        lda divisor
        eor #$FF
        clc
        adc #1
        sta __udiv16.divisor
        lda divisor+1
        eor #$FF
        adc #0
        sta __udiv16.divisor+1
divisorDone

        jsr __udiv16
        bit sign
        bmi negative
        lda __udiv16.dividendAndQuotient
        sta quotient
        lda __udiv16.dividendAndQuotient+1
        sta quotient+1
        rts
negative
        lda __udiv16.dividendAndQuotient
        eor #$FF
        clc
        adc #1
        sta quotient
        lda __udiv16.dividendAndQuotient+1
        eor #$FF
        adc #0
        sta quotient+1
        rts
.endp

; dividend % divisor, signed, as `__smod8`.
.proc __smod16, absolute within 256
        .declare arg i16
dividend .ztemp 2
        .declare arg i16
divisor .ztemp 2
        .declare ret i16
remainder .ztemp 2
sign    .ztemp 1

        lda dividend+1
        sta sign                ; bit 7: the dividend is negative

        lda dividend+1
        bmi dividendNegative
        lda dividend
        sta __umod16.dividend
        lda dividend+1
        sta __umod16.dividend+1
        jmp dividendDone
dividendNegative
        lda dividend
        eor #$FF
        clc
        adc #1
        sta __umod16.dividend
        lda dividend+1
        eor #$FF
        adc #0
        sta __umod16.dividend+1
dividendDone

        lda divisor+1
        bmi divisorNegative
        lda divisor
        sta __umod16.divisor
        lda divisor+1
        sta __umod16.divisor+1
        jmp divisorDone
divisorNegative
        lda divisor
        eor #$FF
        clc
        adc #1
        sta __umod16.divisor
        lda divisor+1
        eor #$FF
        adc #0
        sta __umod16.divisor+1
divisorDone

        jsr __umod16
        bit sign
        bmi negative
        lda __umod16.remainder
        sta remainder
        lda __umod16.remainder+1
        sta remainder+1
        rts
negative
        lda __umod16.remainder
        eor #$FF
        clc
        adc #1
        sta remainder
        lda __umod16.remainder+1
        eor #$FF
        adc #0
        sta remainder+1
        rts
.endp

; The dispatcher over the decoders the program declared, in the order the
; tool numbered them. See docs/spec/transition.md.
.export ngaShowBases
.proc ngaShowBases
        rts
.endp
.export ngaRestore
.proc ngaRestore
        rts
.endp
.export ngaTransform
.proc ngaTransform
        cmp #0
        bne @not0
        jmp diskCopy
@not0
        brk
.endp