Intel 4004 (mcs-4) emulator written from reading manual (not tested)

intel4004.c

// looks correct but not tested at all

	// temp
	unsigned i, t, x;
	// mcs-4 state
	unsigned short pc, stack[3];
	unsigned char r[16], a, c;
	unsigned char test, ram, src, wpm_idx, sp;
	unsigned char rom_out[16], rom_in[16], ram_out[4];
	// 8 banks x 4 chips x 4 registers x (16 + 4) chars x 4 bits
	unsigned char data[8][256], status[8][16][4];

#define NEXT t = prog[pc++]; pc &= 0xfff;
#define ADD a += t; c = a >> 4; a &= 15;

	NEXT

	switch (t & 0xf0) {
	case 0x00:	// 0000-0000 NOP (3.9)
		if (t) goto illegal;
		break;

	case 0x10:	// 0010-cccc JCN (3.7.3)
		i = t & 15;
		t = !test | c << 1 | !a << 2 | 16;
		i = (i & t) + 15;
		NEXT
		if (i & 16) pc = (pc & ~0xff) | t;
		break;

	case 0x20:
		i = t & 14;
		if (t & 1) {
			// 0010-ppp1 SRC (3.10.2)
			src = r[i] << 4 | r[i + 1];
		} else {
			// 0010-ppp0 FIM (3.6.1)
			NEXT
			r[i] = t >> 4;
			r[i + 1] = t & 15;
		}
		break;

	case 0x30:
		i = t & 14;
		x = r[i] << 4 | r[i + 1];
		x |= pc & ~0xff;
		if (t & 1) {
			// 0011-ppp1 JIN (3.7.2)
			pc = x;
		} else {
			// 0011-ppp0 FIN (3.3.2)
			t = prog[x];
			r[i] = t >> 4;
			r[i + 1] = t & 15;
		}
		break;

	case 0x40:	// 0100-xxxx JUN (3.7.1)
		i = t & 15;
		NEXT
		pc = i << 8 | t;
		break;

	case 0x50:	// 0101-xxxx JMS (3.8.1)
		i = t & 15;
		NEXT
		stack[sp] = pc;
		sp = sp < 2 ? sp + 1 : 0;
		pc = i << 8 | t;
		break;

	case 0x60:	// 0110-rrrr INC (3.3.1)
		i = t & 15;
		r[i] = (r[i] + 1) & 15;
		break;

	case 0x70:	// 0111-rrrr ISZ (3.7.4)
		i = t & 15;
		i = r[i] = (r[i] + 1) & 15;
		NEXT
		if (i) pc = (pc & ~0xff) | t;
		break;

	case 0x80;	// 1000-rrrr ADD (3.4.1)
		i = t & 15;
		t = r[i] + c;
		ADD
		break;

	case 0x90;	// 1001-rrrr SUB (3.4.2)
		i = t & 15;
		t = 16 - r[i] - c;
		ADD
		break;

	case 0xa0:	// 1010-rrrr LD (3.4.3)
		i = t & 15;
		a = r[i];
		break;

	case 0xb0:	// 1011-rrrr XCH (3.4.4)
		i = t & 15;
		t = a;
		a = r[i];
		r[i] = t;
		break;

	case 0xc0;	// 1100-xxxx BBL (3.8.2)
		a = t & 15;
		sp = sp ? sp - 1 : 2;
		pc = stack[sp];
		break;

	case 0xd0:	// 1101-xxxx LDM (3.6.2)
		a = t & 15;
		break;

	case 0xe0:	// 1101-xxxx IO (3.11)
		switch (t) {
		case 0xe0:	// 1110-0000 WRM (3.11.4)
			data[ram][src] = a; break;

		case 0xe1:	// 1110-0001 WMP (3.11.6)
			ram_out_out[src >> 6] = a; break;

		case 0xe2:	// 1110-0010 WRR (3.11.7)
			rom_out[src >> 4] = a; break;

		case 0xe3:	// 1110-0011 WPM (3.11.10)
			wpm_idx ^= 4;
			i = rom_out[15] << 8 | src;
			t = prog[i];
			if (rom_out[14]) {
				prog[i] = (t & ~(15 << wpm_idx)) | a << wpm_idx;
			} else {
				rom_in[15 - (wpm_idx >> 3)] = t >> wpm_idx & 15; 
			}
			break;

		case 0xe4:	// 1110-01xx WRn (3.11.5)
		case 0xe5: case 0xe6: case 0xe7:
			status[ram][src >> 4][t & 3] = a; break;

		case 0xe8:	// 1110-1000 SBM (3.11.9)
			t = 16 - data[ram][src] - c; ADD break;

		case 0xe9:	// 1110-1001 RDM (3.11.1)
			a = data[ram][src]; break;

		case 0xea:	// 1110-1010 RDR (3.11.3)
			a = rom_in[src >> 4]; break;

		case 0xeb:	// 1110-1011 ADM (3.11.8)
			t = data[ram][src] + c; ADD break;

		case 0xec:	// 1110-11xx RDn (3.11.2)
		case 0xed: case 0xee: case 0xef:
			a = status[src >> 4][t & 3]; break;
		}
		break;

	case 0xf0:
		switch (t) {
		case 0xf0:	// 1111-0000 CLB (3.5.1)
			a = 0; c = 0; break;

		case 0xf1:	// 1111-0001 CLC (3.5.2)
			c = 0; break;

		case 0xf2:	// 1111-0010 IAC (3.5.3)
			t = 1; ADD break;

		case 0xf3:	// 1111-0011 CMC (3.5.4)
			c ^= 1; break;

		case 0xf4:	// 1111-0011 CMA (3.5.5)
			a ^= 15; break;

		case 0xf5:	// 1111-0101 RAL (3.5.6)
			t = a + c; ADD break;

		case 0xf6:	// 1111-0110 RAR (3.5.7)
			a |= c << 4; c = a & 1; a >>= 1; break;

		case 0xf7:	// 1111-0111 TCC (3.5.8)
			a = c; c = 0; break;

		case 0xf8:	// 1111-1000 DAC (3.5.9)
			t = 15; ADD break;

		case 0xf9:	// 1111-1001 TCS (3.5.10)
			a = 9 + c; c = 0; break;

		case 0xfa:	// 1111-1010 STC (3.5.11)
			c = 1; break;

		case 0xfb:	// 1111-1011 DAA (3.5.12)
			if (a > 9 || c) {
				a += 6; c = 1; a &= 15;
			}
			break;

		case 0xfc: {	// 1111-1100 KBP (3.5.13)
			// a = a ? a & (a - 1) ? 15 : CTZ(a) + 1 : 0;
			const unsigned char kbp_tab[16] =
					{ 0,1,2,15, 3,15,15,15, 4,15,15,15, 15,15,15,15 };
			a = kbp_tab[a]; break;
		}

		case 0xfd:	// 1111-1101 DCL (3.10.1)
			ram = a & 7; break;

		default: goto illegal;
		}
		break;

	default:
	illegal:
	}