Add SBC emulation.

Of dubious quality.

src/65c02isa.csv

@@ -224,34 +224,34 @@ OP_CMP_ABSX,"cmp",ABSOLUTEX,3,emul_cmp,false
 OP_DEC_ABSX,"dec",ABSOLUTEX,3,emul_dec,false
 OP_BBS5_REL,"bbs5",ZPR,2,emul_bbs5,true
 OP_CPX_IMM,"cpx",IMMEDIATE,2,emul_cpx,false
-OP_SBC_IZPX,"sbc",IZPX,2,NULL,false
+OP_SBC_IZPX,"sbc",IZPX,2,emul_sbc,false
 OP_NOPI_E3,"nop",IMMEDIATE,2,NULL,false
 OP_NOPI_E4,"nop",IMPLIED,1,NULL,false
 OP_CPX_ZP,"cpx",ZP,2,emul_cpx,false
-OP_SBC_ZP,"sbc",ZP,2,NULL,false
+OP_SBC_ZP,"sbc",ZP,2,emul_sbc,false
 OP_INC_ZP,"inc",ZP,2,emul_inc,false
 OP_SMB6_ZP,"smb6",ZP,2,emul_smb6,false
 OP_INX,"inx",IMPLIED,1,emul_inx,false
-OP_SBC_IMM,"sbc",IMMEDIATE,2,NULL,false
+OP_SBC_IMM,"sbc",IMMEDIATE,2,emul_sbc,false
 OP_NOP,"nop",IMPLIED,1,emul_nop,false
 OP_NOPI_EC,"nop",IMPLIED,1,NULL,false
 OP_CPX_ABS,"cpx",ABSOLUTE,3,emul_cpx,false
-OP_SBC_ABS,"sbc",ABSOLUTE,3,NULL,false
+OP_SBC_ABS,"sbc",ABSOLUTE,3,emul_sbc,false
 OP_INC_ABS,"inc",ABSOLUTE,3,emul_inc,false
 OP_BBS6_REL,"bbs6",ZPR,2,emul_bbs6,true
 OP_BEQ_REL,"beq",RELATIVE,2,emul_beq,true
-OP_SBC_IZPY,"sbc",IZPY,2,NULL,false
+OP_SBC_IZPY,"sbc",IZPY,2,emul_sbc,false
-OP_SBC_IZP,"sbc",IZP,2,NULL,false
+OP_SBC_IZP,"sbc",IZP,2,emul_sbc,false
 OP_NOPI_F4,"nop",IMPLIED,1,NULL,false
 OP_NOPI_F5,"nop",ZPX,2,NULL,false
-OP_SBC_ZPX,"sbc",ZPX,2,NULL,false
+OP_SBC_ZPX,"sbc",ZPX,2,emul_sbc,false
 OP_INC_ZPX,"inc",ZPX,2,emul_inc,false
 OP_SMB7_ZP,"smb7",ZP,2,emul_smb7,false
 OP_SED,"sed",IMPLIED,1,NULL,false
-OP_SBC_ABSY,"sbc",ABSOLUTEY,3,NULL,false
+OP_SBC_ABSY,"sbc",ABSOLUTEY,3,emul_sbc,false
 OP_PLX,"plx",IMPLIED,1,emul_plx,false
 OP_NOPI_FC,"nop",IMPLIED,1,NULL,false
 OP_NOPI_FD,"nop",ABSOLUTE,3,NULL,false
-OP_SBC_ABSX,"sbc",ABSOLUTEX,3,NULL,false
+OP_SBC_ABSX,"sbc",ABSOLUTEX,3,emul_sbc,false
 OP_INC_ABSX,"inc",ABSOLUTEX,3,emul_inc,false
 OP_BBS7_REL,"bbs7",ZPR,2,emul_bbs7,true

src/emulation.c

@@ -790,6 +790,36 @@ emul_ror(rk65c02emu_t *e, void *id, instruction_t *i)
 	instruction_data_write_1(e, (instrdef_t *) id, i, val);
 }
 
+/* SBC - substract with carry */
+void
+emul_sbc(rk65c02emu_t *e, void *id, instruction_t *i)
+{
+	uint8_t arg;
+	uint16_t res;	/* meh */
+
+	arg = instruction_data_read_1(e, (instrdef_t *) id, i);
+	res = e->regs.A - arg;
+
+	if (e->regs.P & P_CARRY)
+		res--;
+
+	if ((e->regs.A ^ res) & (arg ^ res) & 0x80)
+		e->regs.P |= P_SIGN_OVERFLOW;
+	else
+		e->regs.P &= ~P_SIGN_OVERFLOW;
+
+	/* if the result does not fit into 8 bits then set carry */
+	if (res > 0xFF)
+		e->regs.P |= P_CARRY;
+	else
+		e->regs.P &= ~P_CARRY;
+
+	/* squash the result into accumulator's 8 bits, lol */
+	e->regs.A = (uint8_t) res;
+
+	instruction_status_adjust_zero(e, e->regs.A);
+	instruction_status_adjust_negative(e, e->regs.A);
+}
 /* SEC - set the carry flag */
 void
 emul_sec(rk65c02emu_t *e, void *id, instruction_t *i)