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implement tax and inx and print status each time

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Vineet K 2024-05-24 12:20:04 +05:30
parent 24ca08a83d
commit 8493837ef9
2 changed files with 100 additions and 17 deletions
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3
.gitignore vendored Normal file
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@ -0,0 +1,3 @@
a.out
*.core
*~

114
cpu.c
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@ -1,4 +1,10 @@
#include <stdint.h> #include <stdint.h>
#include <stdio.h>
#define STATUS_UPDATE_ZERO(r) \
(regs.status.zero = r == 0)
#define STATUS_UPDATE_NEGATIVE(r) \
(regs.status.negative = ((r & (1 << 7)) > 0))
struct cpu_flags { struct cpu_flags {
uint8_t carry : 1; uint8_t carry : 1;
@ -12,7 +18,7 @@ struct cpu_flags {
}; };
struct registers { struct registers {
uint8_t acc, x, y, sp; uint8_t a, x, y, sp;
struct cpu_flags status; struct cpu_flags status;
uint16_t pc; uint16_t pc;
}; };
@ -40,28 +46,64 @@ uint8_t addrbus[0x10000];
/* example program taken from https://bugzmanov.github.io/nes_ebook/chapter_3_1.html */ /* example program taken from https://bugzmanov.github.io/nes_ebook/chapter_3_1.html */
uint8_t program[] = { 0xa9, 0xc0, 0xaa, 0xe8, 0x00 }; uint8_t program[] = { 0xa9, 0xc0, 0xaa, 0xe8, 0x00 };
void brk(void) void
brk(void)
{ {
/* $00 */
/* TODO: push regs.pc and regs.status to stack and load IRQ vector */ /* TODO: push regs.pc and regs.status to stack and load IRQ vector */
regs.status.brk = 1; regs.status.brk = 1;
return; return;
} }
void lda(enum addressing_mode mode) void
tax(void)
{
/* $AA */
regs.x = regs.a;
STATUS_UPDATE_ZERO(regs.x);
STATUS_UPDATE_NEGATIVE(regs.x);
}
void
inx(void)
{
/* $E8 */
regs.x++;
STATUS_UPDATE_ZERO(regs.x);
STATUS_UPDATE_NEGATIVE(regs.x);
}
void
lda(enum addressing_mode mode)
{ {
uint8_t val; uint8_t val;
switch (mode) { switch (mode) {
case AM_IMMEDIATE: /* $A9 */ case AM_IMMEDIATE: /* $A9 */
val = program[regs.pc++]; val = program[regs.pc++];
regs.acc = val; printf("arg1 $%02X\n", val);
regs.a = val;
break; break;
case AM_ZERO_PAGE: /* $A5 */ case AM_ZERO_PAGE: /* $A5 */
/* TODO */
break;
case AM_ZERO_PAGE_X: /* $B5 */ case AM_ZERO_PAGE_X: /* $B5 */
/* TODO */
break;
case AM_ABSOLUTE: /* $AD */ case AM_ABSOLUTE: /* $AD */
/* TODO */
break;
case AM_ABSOLUTE_X: /* $BD */ case AM_ABSOLUTE_X: /* $BD */
/* TODO */
break;
case AM_ABSOLUTE_Y: /* $B9 */ case AM_ABSOLUTE_Y: /* $B9 */
/* TODO */
break;
case AM_INDIRECT_X: /* $A1 */ case AM_INDIRECT_X: /* $A1 */
/* TODO */
break;
case AM_INDIRECT_Y: /* $B1 */ case AM_INDIRECT_Y: /* $B1 */
/* TODO */ /* TODO */
break; break;
@ -69,48 +111,86 @@ void lda(enum addressing_mode mode)
return; return;
} }
regs.status.zero = regs.acc == 0; STATUS_UPDATE_ZERO(regs.a);
regs.status.negative = (regs.acc & (1 << 7)) != 0; STATUS_UPDATE_NEGATIVE(regs.a);
} }
void interpret(void) void
interpret(void)
{ {
uint8_t opcode; uint8_t opcode;
regs.pc = 0; regs.pc = 0;
regs.status.unused = 1;
printf("Initial State:\n");
printf("status: %i%i%i%i%i%i%i%i\n", regs.status.carry,
regs.status.zero, regs.status.interrupt_disable,
regs.status.decimal_mode, regs.status.brk,
regs.status.unused, regs.status.overflow,
regs.status.negative);
printf("A: $%02X, X: $%02X, Y: $%02X, SP: $%02X, PC: $%02X\n",
regs.a, regs.x, regs.y, regs.sp, regs.pc);
putchar('\n');
for (;;) { for (;;) {
opcode = program[regs.pc++]; opcode = program[regs.pc++];
printf("opcode: $%02X\n", opcode);
switch (opcode) { switch (opcode) {
case 0x00: case 0x00:
brk(); brk();
return; return;
case 0xa9: case 0xa1:
lda(AM_IMMEDIATE); lda(AM_INDIRECT_X);
break; break;
case 0xa5: case 0xa5:
lda(AM_ZERO_PAGE); lda(AM_ZERO_PAGE);
break; break;
case 0xb5: case 0xa9:
lda(AM_ZERO_PAGE_X); lda(AM_IMMEDIATE);
break;
case 0xaa:
tax();
break; break;
case 0xad: case 0xad:
lda(AM_ABSOLUTE); lda(AM_ABSOLUTE);
break; break;
case 0xbd: case 0xb1:
lda(AM_ABSOLUTE_X); lda(AM_INDIRECT_Y);
break;
case 0xb5:
lda(AM_ZERO_PAGE_X);
break; break;
case 0xb9: case 0xb9:
lda(AM_ABSOLUTE_Y); lda(AM_ABSOLUTE_Y);
break; break;
case 0xa1: case 0xbd:
lda(AM_INDIRECT_X); lda(AM_ABSOLUTE_X);
break; break;
case 0xb1: case 0xe8:
lda(AM_INDIRECT_Y); inx();
break; break;
default: default:
printf("opcode $%02X not implemented\n", opcode);
break; break;
} }
printf("status: %i%i%i%i%i%i%i%i\n", regs.status.carry,
regs.status.zero, regs.status.interrupt_disable,
regs.status.decimal_mode, regs.status.brk,
regs.status.unused, regs.status.overflow,
regs.status.negative);
printf("A: $%02X, X: $%02X, Y: $%02X, SP: $%02X, PC: $%02X\n",
regs.a, regs.x, regs.y, regs.sp, regs.pc);
} }
} }
int
main(void)
{
interpret();
return 0;
}