commit b268be4f7c3fa4f744c62afbb410e58655abcc28
from: Sven M. Hallberg <pesco@khjk.org>
date: Sun Oct 09 14:16:11 2022 UTC

fix jump semantics

Use as destination the value of A as it was at the start of the cycle, before
possibly updating it with the ALU result.

commit - 5261f2c888037eaeb65e345bac5e0dd2deac1874
commit + b268be4f7c3fa4f744c62afbb410e58655abcc28
blob - 4cd01a83f40f2978d90d7269132e5443657caaea
blob + bae1962a2ae432d4efc08de5504c6107c513845c
--- hackem.c
+++ hackem.c
@@ -176,7 +176,7 @@ int
 cpu(uint16_t instr)
 {
 	uint16_t a, comp, ddd, jjj;	/* instruction parts */
-	uint16_t result, y;
+	uint16_t dest, result, y;
 
 	if (bit(instr, 15)) {		/* C-instruction */
 		/* decode instruction */
@@ -185,6 +185,9 @@ cpu(uint16_t instr)
 		comp	= bits(instr, 6, 6);
 		a	= bits(instr, 12, 1);
 
+		/* save jump destination before updating A */
+		dest	= A;
+
 		/* switch ALU input between A and M */
 		if (a)
 			y = readmem(A);
@@ -213,14 +216,14 @@ cpu(uint16_t instr)
 		 *	     0;JMP	     0;JMP
 		 */
 		if (ddd == 0 && jjj == 7 && 
-		    (A == PC || (A == PC - 1 && rom[A] == A)))
+		    (dest == PC || (dest == PC - 1 && rom[dest] == dest)))
 			return 0;	/* stop */
 
 		/* perform jump if required */
 		if ((bit(jjj, 0) && positive(result)) ||
 		    (bit(jjj, 1) && result == 0) ||
 		    (bit(jjj, 2) && negative(result)))
-			PC = A - 1;
+			PC = dest - 1;
 	} else				/* A-instruction */
 		A = instr;
 	PC++;