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CPU update and romdec fix #12

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Merged
rene merged 2 commits into from
Jul 5, 2026
Merged

CPU update and romdec fix #12

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942e725
romdec: Fix NES 2.0 PRG/CHR size and mapper decoding
rene Jul 4, 2026
cd6f557
cpu: Update cpu650x emulator code
rene Jul 5, 2026
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33 changes: 26 additions & 7 deletions src/cpu/cpu650x.c
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Original file line number Diff line number Diff line change
Expand Up @@ -110,6 +110,15 @@ uint8_t cpu_mem[65536];
__stack_data; \
})

/* Read-Modify-Write: some instructions perform a dummy write of the
* unmodified value before writing the modified one, strobing the address
* twice. This might affect memory-mapped I/O devices.
*/
#define RMW_WRITE(addr, current, value) do {\
sbus_write(addr, current); \
sbus_write(addr, value); \
} while (0)

/** Check and perform conditional branch (relative jump) */
#define COND_BRANCH(cond) do {\
uint16_t addr; \
Expand Down Expand Up @@ -268,18 +277,21 @@ static void AND(void)

static void ASL(void)
{
uint8_t orig;

if (addrmode == mIMP) {
SET_PFLAG(CPU.P.flags.C, (CPU.A & 0x80) != 0);
CPU.A = (CPU.A << 1) & 0xfe;
SET_PFLAG(CPU.P.flags.N, (CPU.A & 0x80) != 0);
SET_PFLAG(CPU.P.flags.Z, CPU.A == 0);
} else {
FETCH_DATA();
orig = fetched_data;
SET_PFLAG(CPU.P.flags.C, (fetched_data & 0x80) != 0);
fetched_data = (fetched_data << 1) & 0xfe;
SET_PFLAG(CPU.P.flags.N, (fetched_data & 0x80) != 0);
SET_PFLAG(CPU.P.flags.Z, fetched_data == 0);
sbus_write(fetched_addr, fetched_data);
RMW_WRITE(fetched_addr, orig, fetched_data);
}
}

Expand Down Expand Up @@ -413,8 +425,8 @@ static void DEC(void)
uint8_t res;

FETCH_DATA();
res = fetched_data - 1;
sbus_write(fetched_addr, res);
res = fetched_data - 1;
RMW_WRITE(fetched_addr, fetched_data, res);
SET_PFLAG(CPU.P.flags.N, (res & 0x80) != 0);
SET_PFLAG(CPU.P.flags.Z, res == 0);
}
Expand Down Expand Up @@ -447,7 +459,7 @@ static void INC(void)

FETCH_DATA();
res = fetched_data + 1;
sbus_write(fetched_addr, res);
RMW_WRITE(fetched_addr, fetched_data, res);
SET_PFLAG(CPU.P.flags.N, (res & 0x80) != 0);
SET_PFLAG(CPU.P.flags.Z, res == 0);
}
Expand Down Expand Up @@ -509,18 +521,21 @@ static void LDY(void)

static void LSR(void)
{
uint8_t orig;

if (addrmode == mIMP) {
SET_PFLAG(CPU.P.flags.C, (CPU.A & 0x1) != 0);
CPU.A = ((CPU.A >> 1) & 0x7f);
CPU.P.flags.N = 0;
SET_PFLAG(CPU.P.flags.Z, CPU.A == 0);
} else {
FETCH_DATA();
orig = fetched_data;
SET_PFLAG(CPU.P.flags.C, (fetched_data & 0x1) != 0);
fetched_data = ((fetched_data >> 1) & 0x7f);
CPU.P.flags.N = 0;
SET_PFLAG(CPU.P.flags.Z, fetched_data == 0);
sbus_write(fetched_addr, fetched_data);
RMW_WRITE(fetched_addr, orig, fetched_data);
}
}

Expand Down Expand Up @@ -561,6 +576,7 @@ static void PLP(void)

static void ROL(void)
{
uint8_t orig;
uint8_t tmp = CPU.P.flags.C;

if (addrmode == mIMP) {
Expand All @@ -570,16 +586,18 @@ static void ROL(void)
SET_PFLAG(CPU.P.flags.Z, CPU.A == 0);
} else {
FETCH_DATA();
orig = fetched_data;
SET_PFLAG(CPU.P.flags.C, (fetched_data & 0x80) != 0);
fetched_data = ((fetched_data << 1) & 0xfe) | tmp;
SET_PFLAG(CPU.P.flags.N, (fetched_data & 0x80) != 0);
SET_PFLAG(CPU.P.flags.Z, fetched_data == 0);
sbus_write(fetched_addr, fetched_data);
RMW_WRITE(fetched_addr, orig, fetched_data);
}
}

static void ROR(void)
{
uint8_t orig;
uint8_t tmp = CPU.P.flags.C;

if (addrmode == mIMP) {
Expand All @@ -589,12 +607,13 @@ static void ROR(void)
SET_PFLAG(CPU.P.flags.Z, CPU.A == 0);
} else {
FETCH_DATA();
orig = fetched_data;
SET_PFLAG(CPU.P.flags.C, (fetched_data & 0x1) != 0);
fetched_data = ((fetched_data >> 1) & 0x7f)
| ((tmp << 7) & 0x80);
CPU.P.flags.N = tmp;
SET_PFLAG(CPU.P.flags.Z, fetched_data == 0);
sbus_write(fetched_addr, fetched_data);
RMW_WRITE(fetched_addr, orig, fetched_data);
}
}

Expand Down
12 changes: 7 additions & 5 deletions src/romdec.c
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Original file line number Diff line number Diff line change
Expand Up @@ -98,9 +98,10 @@ int load_rom(const char *pathname, rom_t **rom)
romf->chr_size = romf->header->chr_size * (8 * 1024);
} else {
if (romf->header->flag9.nes.prgrom_size < 0xf) {
/* Size in 16 KB units: MSB nibble (flag9) + LSB byte */
romf->prg_size =
((romf->header->flag9.nes.prgrom_size << 4) & 0xf0) |
(romf->header->prg_size & 0x0f);
((romf->header->flag9.nes.prgrom_size << 8) & 0xf00) |
romf->header->prg_size;
romf->prg_size *= (16 * 1024);
} else {
/* Get exponent and multiplier */
Expand All @@ -111,9 +112,10 @@ int load_rom(const char *pathname, rom_t **rom)
}

if (romf->header->flag9.nes.chrrom_size < 0xf) {
/* Size in 8 KB units: MSB nibble (flag9) + LSB byte */
romf->chr_size =
((romf->header->flag9.nes.chrrom_size << 4) & 0xf0) |
(romf->header->prg_size & 0x0f);
((romf->header->flag9.nes.chrrom_size << 8) & 0xf00) |
romf->header->chr_size;
romf->chr_size *= (8 * 1024);
} else {
/* Get exponent and multiplier */
Expand Down Expand Up @@ -142,7 +144,7 @@ int load_rom(const char *pathname, rom_t **rom)
romf->mapper = ((romf->header->flag7.ines.mapper << 4) & 0xf0) |
(romf->header->flag6.nes.mapper & 0xf);
} else {
romf->mapper = ((romf->header->flag8.raw << 8) & 0xff00) |
romf->mapper = ((romf->header->flag8.nes.mapper << 8) & 0xf00) |
((romf->header->flag7.nes.mapper << 4) & 0xf0) |
(romf->header->flag6.nes.mapper & 0xf);
}
Expand Down
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