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Fix V (overflow) flag calculation in ADC and SBC #2

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Fix V (overflow) flag calculation in ADC and SBC #2

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1139572
Fix SBC carry flag calculation
garry-jeromson Jan 30, 2026
89ae6f5
Fix V (overflow) flag calculation in ADC and SBC
garry-jeromson Feb 6, 2026
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38 changes: 31 additions & 7 deletions src/cpu/65816-ops.c
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Original file line number Diff line number Diff line change
Expand Up @@ -34,7 +34,10 @@ void i_adc(CPU_t *cpu, memory_t *mem, uint8_t size, uint8_t cycles, CPU_Addr_Mod
else // Binary mode
{
al = (cpu->C & 0xff) + val + cpu->P.C;
cpu->P.V = ((int16_t)al < -128 || (int16_t)al > 127) ? 1 : 0;
// V flag: check if signed result overflows 8-bit signed range
// Must sign-extend operands before computing to get correct signed result
int16_t signed_al = (int8_t)(cpu->C & 0xff) + (int8_t)val + cpu->P.C;
cpu->P.V = (signed_al < -128 || signed_al > 127) ? 1 : 0;
}

// 1f
Expand Down Expand Up @@ -93,7 +96,10 @@ void i_adc(CPU_t *cpu, memory_t *mem, uint8_t size, uint8_t cycles, CPU_Addr_Mod
else
{
al = cpu->C + val + cpu->P.C;
cpu->P.V = ((int32_t)al < -32768 || (int32_t)al > 32767) ? 1 : 0;
// V flag: check if signed result overflows 16-bit signed range
// Must sign-extend operands before computing to get correct signed result
int32_t signed_al = (int16_t)cpu->C + (int16_t)val + cpu->P.C;
cpu->P.V = (signed_al < -32768 || signed_al > 32767) ? 1 : 0;
}
// 1f
cpu->C = al & 0xffff;
Expand Down Expand Up @@ -2266,6 +2272,8 @@ void i_sbc(CPU_t *cpu, memory_t *mem, uint8_t size, uint8_t cycles, CPU_Addr_Mod
{
uint8_t val = _get_mem_byte(mem, addr, cpu->setacc);
uint16_t al, alb;
uint8_t original_a = cpu->C & 0xff; // Save original A for carry calculation
uint8_t original_c = cpu->P.C; // Save original carry for carry calculation

// Binary mode calculation
alb = (cpu->C & 0xff) - val + cpu->P.C - 1;
Expand Down Expand Up @@ -2299,15 +2307,24 @@ void i_sbc(CPU_t *cpu, memory_t *mem, uint8_t size, uint8_t cycles, CPU_Addr_Mod
cpu->P.Z = ((al & 0xff) == 0) ? 1 : 0;

// C and V are based on the binary result
cpu->P.V = ((int16_t)alb < -128 || (int16_t)alb > 127) ? 1 : 0;
cpu->P.C = (alb >= 0x100) ? 1 : 0;
// V flag: check if signed result overflows 8-bit signed range
// Must sign-extend operands before computing to get correct signed result
{
int16_t signed_alb = (int8_t)original_a - (int8_t)val + original_c - 1;
cpu->P.V = (signed_alb < -128 || signed_alb > 127) ? 1 : 0;
}
// For SBC, carry is set if no borrow occurred: A >= M + (1 - C_in)
// Using original values saved at start of function
cpu->P.C = ((uint16_t)original_a + original_c > val) ? 1 : 0;

}
else // 16-bit
{
uint16_t val;
uint32_t al, alb;

uint16_t original_a = cpu->C; // Save original A for carry calculation
uint8_t original_c = cpu->P.C; // Save original carry for carry calculation

if (mode == CPU_ADDR_DP || mode == CPU_ADDR_DPX ||
mode == CPU_ADDR_IMMD || mode == CPU_ADDR_SR)
{
Expand Down Expand Up @@ -2364,8 +2381,15 @@ void i_sbc(CPU_t *cpu, memory_t *mem, uint8_t size, uint8_t cycles, CPU_Addr_Mod
cpu->P.Z = ((al & 0xffff) == 0) ? 1 : 0;

// C and V are based on the binary result
cpu->P.V = ((int32_t)alb < -32768 || (int32_t)alb > 32767) ? 1 : 0;
cpu->P.C = (alb >= 0x10000) ? 1 : 0;
// V flag: check if signed result overflows 16-bit signed range
// Must sign-extend operands before computing to get correct signed result
{
int32_t signed_alb = (int16_t)original_a - (int16_t)val + original_c - 1;
cpu->P.V = (signed_alb < -32768 || signed_alb > 32767) ? 1 : 0;
}
// For SBC, carry is set if no borrow occurred: A >= M + (1 - C_in)
// Using original values saved at start of function
cpu->P.C = ((uint32_t)original_a + original_c > val) ? 1 : 0;

cpu->cycles += 1;
if (mode == CPU_ADDR_IMMD)
Expand Down