109 lines
3.7 KiB
Python
109 lines
3.7 KiB
Python
"""Build a Sega Master System .sms cartridge: a Z80 VDP-setup program + data.
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The Z80 code (org $0000) programs the VDP for mode 4 (256x192), uploads the tile
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patterns to VRAM $0000, the name table to $3800 and the palette to CRAM, turns
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the display on, then idles. A valid "TMR SEGA" header + checksum is written at
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$7FF0 so the export BIOS accepts the cartridge.
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"""
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from __future__ import annotations
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from .z80 import Asm
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ROM_SIZE = 0x8000 # 32 KB (maps to $0000-$7FFF, no mapper needed)
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PORT_CTRL = 0xBF
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PORT_DATA = 0xBE
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# VDP registers 0-10 for a plain 256x192 mode-4 background screen. Reg 1 starts
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# with the display OFF ($80); it is set to $C0 (display on) at the end.
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VDP_REGS = [0x04, 0x80, 0xFF, 0xFF, 0xFF, 0xFF, 0xFB, 0x00, 0x00, 0x00, 0xFF]
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def _code(reglen, patlen, ntlen) -> bytes:
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a = Asm(0x0000)
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a.di()
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a.im1()
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a.ld_sp(0xDFF0)
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# --- VDP register init: write VDP_REGS[i] then $80|i ---
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a.ld_hl("REGDATA")
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a.ld_b(len(VDP_REGS))
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a.ld_c(0x00)
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a.label("rinit")
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a.ld_a_hl(); a.out_a(PORT_CTRL) # value
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a.ld_a_c(); a.or_n(0x80); a.out_a(PORT_CTRL) # $80 | reg
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a.inc_hl(); a.inc_c()
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a.djnz("rinit")
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# --- upload patterns to VRAM $0000 ---
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a.xor_a(); a.out_a(PORT_CTRL)
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a.ld_a(0x40); a.out_a(PORT_CTRL) # $0000 | write
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a.ld_hl("PATDATA")
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a.ld_bc(patlen)
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a.label("ptile")
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a.ld_a_hl(); a.out_a(PORT_DATA)
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a.inc_hl(); a.dec_bc()
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a.ld_a_b(); a.or_c(); a.jp_nz("ptile")
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# --- upload name table to VRAM $3800 ---
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a.xor_a(); a.out_a(PORT_CTRL)
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a.ld_a(0x78); a.out_a(PORT_CTRL) # $3800 | write ($38 | $40)
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a.ld_hl("NTDATA")
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a.ld_bc(ntlen)
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a.label("pnt")
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a.ld_a_hl(); a.out_a(PORT_DATA)
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a.inc_hl(); a.dec_bc()
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a.ld_a_b(); a.or_c(); a.jp_nz("pnt")
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# --- upload palette to CRAM 0 ---
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a.xor_a(); a.out_a(PORT_CTRL)
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a.ld_a(0xC0); a.out_a(PORT_CTRL) # CRAM write
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a.ld_hl("PALDATA")
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a.ld_b(0x20) # 32 colours
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a.label("ppal")
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a.ld_a_hl(); a.out_a(PORT_DATA)
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a.inc_hl()
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a.djnz("ppal")
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# --- disable sprites: write $D0 (list terminator) to SAT Y[0] at $3F00 ---
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a.xor_a(); a.out_a(PORT_CTRL)
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a.ld_a(0x7F); a.out_a(PORT_CTRL) # $3F00 | write
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a.ld_a(0xD0); a.out_a(PORT_DATA)
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# --- display on (reg 1 = $C0) ---
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a.ld_a(0xC0); a.out_a(PORT_CTRL)
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a.ld_a(0x81); a.out_a(PORT_CTRL)
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a.label("hang")
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a.jp("hang")
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# data labels live right after the code, in this order
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base = len(a.code)
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a.set_label("REGDATA", base)
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a.set_label("PATDATA", base + reglen)
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a.set_label("NTDATA", base + reglen + patlen)
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a.set_label("PALDATA", base + reglen + patlen + ntlen)
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return a.resolve()
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def build_rom(patterns: bytes, nametable: bytes, palette: bytes) -> bytes:
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code = _code(len(VDP_REGS), len(patterns), len(nametable))
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rom = bytearray(ROM_SIZE)
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blob = code + bytes(VDP_REGS) + bytes(patterns) + bytes(nametable) + bytes(palette)
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if len(blob) > 0x7FF0:
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raise ValueError("SMS image data overruns the 32KB cartridge")
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rom[0:len(blob)] = blob
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# "TMR SEGA" header at $7FF0 (export region, 32KB) so the BIOS accepts it.
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rom[0x7FF0:0x7FF8] = b"TMR SEGA"
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rom[0x7FF8:0x7FFA] = bytes(2) # reserved
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chk = sum(rom[0:0x7FF0]) & 0xFFFF # checksum of $0000-$7FEF
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rom[0x7FFA] = chk & 0xFF
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rom[0x7FFB] = (chk >> 8) & 0xFF
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rom[0x7FFC:0x7FFF] = bytes(3) # product code / version
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rom[0x7FFF] = 0x4C # region 4 (export) | size $C (32K)
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return bytes(rom)
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def write_sms(rom: bytes, path: str) -> str:
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with open(path, "wb") as f:
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f.write(rom)
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return path
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