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The Dust Council
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1
lenser/a2600/viewer/__init__.py
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lenser/a2600/viewer/__init__.py
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138
lenser/a2600/viewer/a2600.s
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lenser/a2600/viewer/a2600.s
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; lenser -- Atari 2600 (VCS) image viewer kernel (6502, "racing the beam").
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;
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; No framebuffer -- per visible scanline the kernel feeds the TIA a 40-pixel
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; asymmetric playfield (left 20 + right 20, rewritten mid-line) plus a shared
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; playfield colour (COLUPF) and two backgrounds -- COLUBK is set for the left
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; half during HBLANK then rewritten mid-line for the right half, so each
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; scanline shows 3 colours (left bg + right bg + foreground). The nine 192-byte
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; data tables are page-aligned (set by the cartridge builder) so LDA tab,Y never
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; crosses a page and the kernel stays cycle-exact.
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;
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; #defines from the wrapper -- PF0L,PF1L,PF2L,PF0R,PF1R,PF2R,BKL,BKR,PFT (table
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; bases), WAITMODE (0 forever / 1 fire / 2 seconds), WAITSECS.
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;
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; assembled by a2600/viewer/assemble.py via xa
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VSYNC = $00
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VBLANK = $01
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WSYNC = $02
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NUSIZ0 = $04
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COLUPF = $08
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COLUBK = $09
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CTRLPF = $0A
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PF0 = $0D
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PF1 = $0E
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PF2 = $0F
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INPT4 = $3C
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FRLO = $80 ; frame counter (seconds mode)
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FRHI = $81
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PARITY = $82 ; interlace frame parity (IL mode)
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* = $f000
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start:
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sei
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cld
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ldx #0
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txa
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clr:
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sta $00,x
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inx
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bne clr ; clear $00-$FF (RAM + TIA)
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ldx #$ff
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txs
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frame:
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lda #2
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sta VSYNC
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sta WSYNC
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sta WSYNC
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sta WSYNC ; 3 VSYNC lines
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lda #0
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sta VSYNC
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lda #2
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sta VBLANK
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sta CTRLPF ; reflect off (we draw both halves explicitly)
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lda #0
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sta CTRLPF
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#if IL
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; temporal interlace -- alternate the two 4K banks (frame A / frame B)
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; each frame; the kernel is identical in both banks so execution
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; continues seamlessly and only the data tables differ.
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lda PARITY
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eor #1
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sta PARITY
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beq selbank0
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lda $1ff9 ; F8 hotspot -> select bank 1 (AD F9 1F)
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jmp bankdone
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selbank0:
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lda $1ff8 ; F8 hotspot -> select bank 0 (AD F8 1F)
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bankdone:
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#endif
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ldx #36
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vbl:
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sta WSYNC
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dex
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bne vbl
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lda #0
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sta VBLANK
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; ---- visible image, 192 cycle-exact scanlines ----
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ldy #0
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kloop:
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sta WSYNC
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lda BKL,y ; left background (HBLANK)
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sta COLUBK
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lda PFT,y ; shared foreground
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sta COLUPF
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lda PF0L,y
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sta PF0
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lda PF1L,y
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sta PF1
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lda PF2L,y
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sta PF2
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lda PF0R,y ; right playfield PF0 (early, before mid-line)
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sta PF0
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lda BKR,y ; right background (lands at the half boundary)
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sta COLUBK
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lda PF1R,y
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sta PF1
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lda PF2R,y
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sta PF2
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iny
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cpy #192
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bne kloop
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lda #0
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sta PF0
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sta PF1
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sta PF2
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sta COLUBK
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; ---- hold / exit ----
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#if WAITMODE == 1
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lda INPT4
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bmi over ; bit7 set = fire not pressed
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jmp start ; pressed -> reset (re-display)
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#endif
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#if WAITMODE == 2
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inc FRLO
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bne fr_ok
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inc FRHI
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fr_ok:
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lda FRHI
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cmp #>(WAITSECS*60)
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bcc over
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lda FRLO
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cmp #<(WAITSECS*60)
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bcc over
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jmp start
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#endif
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over:
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lda #2
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sta VBLANK
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ldx #30
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ovl:
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sta WSYNC
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dex
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bne ovl
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jmp frame
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92
lenser/a2600/viewer/assemble.py
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lenser/a2600/viewer/assemble.py
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"""Assemble the Atari 2600 kernel with `xa` and lay out the cartridge.
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A static image is a 4K cart (one kernel + nine 192-byte tables). An interlace
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image is an 8K F8-bankswitch cart: two 4K banks, each holding the SAME kernel
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plus one of the two table sets (frame A / frame B). The kernel toggles the bank
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once per frame, so the two frames alternate at 60Hz and blend to ~4-6 perceived
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colours per scanline.
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"""
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from __future__ import annotations
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import os
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import shutil
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import subprocess
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import tempfile
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VIEWER_DIR = os.path.dirname(os.path.abspath(__file__))
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WAIT_MODES = {"forever": 0, "fire": 1, "key": 1, "seconds": 2}
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# page-aligned data tables ($F100..$F900)
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TABLES = {"PF0L": 0xF100, "PF1L": 0xF200, "PF2L": 0xF300, "PF0R": 0xF400,
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"PF1R": 0xF500, "PF2R": 0xF600, "BKL": 0xF700, "BKR": 0xF800,
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"PFT": 0xF900}
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ORDER = ["PF0L", "PF1L", "PF2L", "PF0R", "PF1R", "PF2R", "BKL", "BKR", "PFT"]
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class AssemblerError(RuntimeError):
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pass
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def have_xa() -> bool:
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return shutil.which("xa") is not None
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def _assemble_kernel(display: str, seconds: int, interlace: bool) -> bytes:
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if not have_xa():
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raise AssemblerError("The 'xa' (xa65) assembler was not found on PATH.\n"
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"Install it with: sudo apt install xa65")
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waitmode = WAIT_MODES.get(display, 0)
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defs = "".join(f"#define {n} ${a:04X}\n" for n, a in TABLES.items())
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wrapper = (defs +
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f"#define WAITMODE {waitmode}\n"
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f"#define WAITSECS {max(0, int(seconds))}\n"
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f"#define IL {1 if interlace else 0}\n"
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'#include "a2600.s"\n')
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with tempfile.TemporaryDirectory() as td:
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out = os.path.join(td, "k.bin")
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fd, wrap = tempfile.mkstemp(suffix=".s", prefix="_wrap_", dir=VIEWER_DIR)
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try:
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with os.fdopen(fd, "w") as f:
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f.write(wrapper)
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proc = subprocess.run(["xa", "-o", out, os.path.basename(wrap)],
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capture_output=True, text=True, cwd=VIEWER_DIR)
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if proc.returncode != 0:
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raise AssemblerError(f"xa failed:\n{proc.stdout}{proc.stderr}")
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with open(out, "rb") as f:
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kernel = f.read()
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finally:
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os.unlink(wrap)
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if len(kernel) > 0x100:
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raise AssemblerError(f"kernel is {len(kernel)} bytes, overruns the table "
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"page at $F100")
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return kernel
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def _lay_bank(kernel: bytes, data: bytes) -> bytearray:
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"""One 4096-byte bank: kernel at $F000, nine tables, reset/IRQ vectors."""
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rom = bytearray(b"\x00" * 4096)
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rom[0:len(kernel)] = kernel
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tables = [data[i * 192:(i + 1) * 192] for i in range(9)]
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for name, tab in zip(ORDER, tables):
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off = TABLES[name] - 0xF000
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rom[off:off + 192] = tab
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rom[0xFFC] = 0x00 # reset vector lo -> $F000
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rom[0xFFD] = 0xF0
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rom[0xFFE] = 0x00 # IRQ/BRK vector
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rom[0xFFF] = 0xF0
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return rom
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def build_cart(data: bytes, display: str = "forever", seconds: int = 0) -> bytes:
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"""data = nine 192-byte tables (static, 4K cart) or eighteen (two sets ->
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interlace 8K F8 cart). Returns the cart image with reset vectors set."""
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if len(data) == 9 * 192:
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kernel = _assemble_kernel(display, seconds, interlace=False)
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return bytes(_lay_bank(kernel, data))
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if len(data) == 18 * 192:
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kernel = _assemble_kernel(display, seconds, interlace=True)
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bank0 = _lay_bank(kernel, data[:9 * 192])
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bank1 = _lay_bank(kernel, data[9 * 192:])
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return bytes(bank0 + bank1)
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raise ValueError(f"expected 1728 or 3456 bytes of tables, got {len(data)}")
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