fix SBC bug in the rewrite and improve logging

1 files changed, 29 insertions, 60 deletions

diff --git a/cpu.c b/cpu.c
index 6e44c7f..df9c852 100644
--- a/cpu.c
+++ b/cpu.c
@@ -99,55 +99,6 @@ memwrite16(uint16_t addr, uint16_t word)
         memory[addr + 1] = (word & 0xFF00) >> 8;
 }
 
-static uint8_t
-opcode_arg(enum addressing_mode mode)
-{
-        uint16_t arg, val;
-
-        if (mode != AM_ABS && mode != AM_ABS_X && mode != AM_ABS_Y) {
-                arg = peek(regs.pc++);
-        } else {
-                arg = peek16(regs.pc);
-                regs.pc += 2;
-        }
-
-        switch (mode) {
-        case AM_IMM:
-        case AM_REL:
-                val = arg;
-                break;
-        case AM_ZP:
-                val = peek(arg % 256);
-                break;
-        case AM_ZP_X:
-                val = peek((arg + regs.x) % 256);
-                break;
-        case AM_ZP_Y:
-                val = peek((arg + regs.y) % 256);
-                break;
-        case AM_ABS:
-                val = peek16(arg);
-                break;
-        case AM_ABS_X:
-                val = peek16(arg + regs.x);
-                break;
-        case AM_ABS_Y:
-                val = peek16(arg + regs.y);
-                break;
-        case AM_IND_X:
-                val = peek(peek((arg + regs.x) % 256) + peek((arg + regs.x + 1) % 256) * 256);
-                break;
-        case AM_IND_Y:
-                val = peek(peek(arg) + peek((arg + 1) % 256) * 256 + regs.y);
-                break;
-        default:
-                fprintf(stderr, "INVALID ADDRESSING MODE %i\n", mode);
-                abort();
-        }
-
-        return val;
-}
-
 static uint16_t
 opcode_mem(enum addressing_mode mode)
 {
@@ -193,7 +144,7 @@ opcode_mem(enum addressing_mode mode)
                 val = peek(arg) + peek((arg + 1) % 256) * 256 + regs.y;
                 break;
         default:
-                fprintf(stderr, "opcode_mem INVALID ADDRESSING MODE %i\n", mode);
+                fprintf(stderr, "INVALID ADDRESSING MODE %i\n", mode);
                 abort();
         }
 
@@ -271,7 +222,8 @@ BEQ(uint16_t arg)
 void
 BIT(uint16_t arg)
 {
-        uint8_t tmp = peek(arg);
+        uint8_t tmp = arg;
+
         regs.status.zero = (regs.a & tmp) == 0;
         regs.status.overflow = (tmp & (1 << 6)) != 0;
         STATUS_UPDATE_NEGATIVE(tmp);
@@ -628,8 +580,18 @@ RTS(uint16_t arg)
 void
 SBC(uint16_t arg)
 {
-        /* SBC is described online as ADC with argument as two's complement */
-        ADC(~(uint8_t)arg);
+        uint8_t tmp = arg & 0xFF;
+        uint16_t diff;
+
+        diff = regs.a - tmp - !regs.status.carry;
+
+        regs.status.carry = diff > 0xFF;
+        regs.status.carry = !regs.status.carry;
+        /* overflow flag formula: https://stackoverflow.com/a/29224684 */
+        regs.status.overflow = ((regs.a ^ arg) & (regs.a ^ diff) & 0x80) != 0;
+        regs.a = diff & 0xFF;
+
+        STATUS_UPDATE_NZ(regs.a);
 }
 
 void
@@ -759,10 +721,10 @@ interpret(void)
                         printf("$%04X", arg);
                         break;
                 case AM_ABS_X:
-                        printf("$%04X,X", arg);
+                        printf("$%04X,X @ %04X", arg - regs.x, arg);
                         break;
                 case AM_ABS_Y:
-                        printf("$%04X,Y", arg);
+                        printf("$%04X,Y @ %04X", arg - regs.y, arg);
                         break;
                 case AM_IND:
                         printf("($%04X) = %04X\t\t", arg, peek16(arg));
@@ -776,13 +738,20 @@ interpret(void)
                         break;
                 }
 
-                if (opcodes[op].didmemory) printf(" = %02X", peek(arg));
-                printf("\t\t\t");
+                if (opcodes[op].memread || opcodes[op].memwrite)
+                        printf(" = %02X", peek(arg));
+
+                if (mode != AM_ABS_X && mode != AM_ABS_Y)
+                        putchar('\t');
+                printf("\t\t");
 
-                printf("A:%02X X:%02X Y:%02X P:%02X SP:%02X CYC:%d %08b\n",
-                    regs.a, regs.x, regs.y, STATUS_TO_INT(), regs.sp, cycles, STATUS_TO_INT());
+                printf("A:%02X X:%02X Y:%02X P:%02X SP:%02X CYC:%d\n",
+                    regs.a, regs.x, regs.y, STATUS_TO_INT(), regs.sp, cycles);
 
-                opcodes[op].instr(arg);
+                if (opcodes[op].memread)
+                        opcodes[op].instr(peek(arg));
+                else
+                        opcodes[op].instr(arg);
                 cycles += opcodes[op].cycles;
         }
 loop_exit: