diff --git a/gr-sim/6502_emulate.c b/gr-sim/6502_emulate.c
index 4d81b8a9..9ad47a92 100644
--- a/gr-sim/6502_emulate.c
+++ b/gr-sim/6502_emulate.c
@@ -68,9 +68,11 @@ void sbc(int value) {
 	int temp_value;
 
 	temp_a=a&0xff;
-	temp_value=value&0xff;
+	temp_value=(~value)&0xff;
 
-	result=temp_a-temp_value-(!c);
+	result=temp_a+temp_value+c;
+
+//	printf("SBC: %x - %x (%x) = %x\n",a,value,c,result);
 
 	c=(result&0x100)>>8;
 	n=(result&0x80)>>7;
@@ -79,6 +81,8 @@ void sbc(int value) {
 
 	a=result&0xff;
 	z=(a==0);
+
+
 }
 
 void cmp(int value) {
diff --git a/gr-sim/6502_test.c b/gr-sim/6502_test.c
index b2448649..d0aa0262 100644
--- a/gr-sim/6502_test.c
+++ b/gr-sim/6502_test.c
@@ -54,7 +54,7 @@ static void test_sbc(void) {
 			if (a!=((i-j-0)&0xff)) {
 				printf("SBC: Error!  %d-%d should be %d, not %d\n",i,j,i-j-0,a);
 			}
-			if (c!=(((i-j-0)>>8)&0x1)) {
+			if (c==(((i-j-0)>>8)&0x1)) {
 				printf("SBC: Error!  Carry should be %d, not %d\n",((i-j-0)>>8)&0x1,c);
 			}
 			if ((a==0) && (z!=1)) printf("SBC error, zflag wrong\n");
@@ -71,7 +71,7 @@ static void test_sbc(void) {
 			if (a!=((i-j-1)&0xff)) {
 				printf("SBC: Error!  %d-%d should be %d, not %d\n",i,j,i-j-1,a);
 			}
-			if (c!=(((i-j-1)>>8)&0x1)) {
+			if (c==(((i-j-1)>>8)&0x1)) {
 				printf("SBC: Error!  Carry should be %d, not %d\n",((i-j-1)>>8)&0x1,c);
 			}
 			if ((a==0) && (z!=1)) printf("SBC error, zflag wrong\n");