diff options
Diffstat (limited to 'chip8.c')
| -rw-r--r-- | chip8.c | 25 |
1 files changed, 20 insertions, 5 deletions
@@ -133,7 +133,7 @@ chip8_beep() void unknown_opcode(uint16_t bad_opcode) { - fprintf(stderr, "unknown opcode: 0x%02X at 0x%02X\n", bad_opcode, PC); + fprintf(stderr, "unknown opcode: 0x%02X at 0x%02X\n", bad_opcode, PC-2); // minus 2 because we increment PC by 2 at the start of decoding } void chip8_cycle() @@ -145,7 +145,7 @@ chip8_cycle() /* * the opcode is a 2 byte value, while our memory is 1 byte * so to get our opcode we need to combine 2 bytes located at PC and PC+1 - * we right shift the first byte by 8 (1 byte) to place it in the high byte + * we left shift the first byte by 8 (1 byte) to place it in the high byte * and we store the second byte in the lower byte */ opcode = (memory[PC] << 8) | memory[PC+1]; @@ -155,7 +155,8 @@ chip8_cycle() uint8_t x = (opcode >> 8) & 0x000F; // lower 4 bits of the high byte, we discard the low byte by right shifting it out uint8_t y = (opcode >> 4) & 0x000F; // upper 4 bits of the low byte, so we need to discard the lower 4 bits uint8_t kk = opcode & 0x00FF; // lowest 8 bits - + printf("parsing at 0x%03X\n", PC); + PC += 2; // TODO: remove switch (opcode & 0xF000) { /* @@ -172,17 +173,32 @@ chip8_cycle() draw_flag = 1; break; case 0x00EE: /* ret (return from subroutine) */ + // TODO: test, check for out of bounds? + PC = stack[--SP]; break; default: unknown_opcode(opcode); } break; } + case 0x1000: /*JP addr (jump to memory address nnn) */ + PC = nnn; + break; + case 0x2000: /* CALL addr (call subroutine at addr, increment SP and put current PC on top of stack, set PC to nnn) */ + // TODO: test, bounds check? + stack[SP++] = PC; + PC = nnn; + break; case 0x6000: /* LD Vx, byte (load byte in register x) */ { V[x] = kk; break; } - + case 0x7000: /* ADD Vx, byte (add byte to Vx and store result in Vx) */ + V[x] = V[x] + kk; + break; + case 0x8000: /* LD Vx, Vy (load Vy into Vx register) */ + V[x] = V[y]; + break; default: unknown_opcode(opcode); } @@ -194,5 +210,4 @@ chip8_cycle() chip8_beep(); sound_timer--; } - PC += 2; } |