Implement subWord, stSub, stGtEq in calculator.asm

calculator.asm

@@ -35,17 +35,27 @@ stAdd:
 
 ; a b -- a-b
 stSub:
-	cleq r0, r0, stNeg
-	breq r0, r0, stAdd
-
-; n -- -n
-stNeg:
 	cleq r0, r0, popWord
-	nand r0, r0
-	nand r1, r1
-	cleq r0, r0, incWord
+	cleq r0, r0, tmpStoreWord01
+	cleq r0, r0, popWord
+	cleq r0, r0, tmpLoadWord23
+	cleq r0, r0, subWord
 	breq r0, r0, pushWord
 
+; a b --
+; if a >= b then r0:r1 = 0001
+; else r0:r1 = 0000
+stGtEq:
+	cleq r0, r0, popWord
+	cleq r0, r0, tmpStoreWord01
+	cleq r0, r0, popWord
+	cleq r0, r0, tmpLoadWord23
+	cleq r0, r0, subWord
+	xor r0, r0
+	xor r1, r1
+	or r1, r2
+	ret
+
 ; ------------------------------------------------------------------
 ; I/O
 ; ------------------------------------------------------------------
@@ -228,6 +238,45 @@ addWord:
 	addWordSecondHigh: data 0
 	addWordResultLow: data 0
 
+; in:
+;  r0:r1 = minuend
+;  r2:r3 = subtrahend
+; out:
+;  r0:r1 = result
+;  r2 = carryout
+; trashes:
+;  r3
+subWord:
+	store subWordMinuendHigh, r0
+	store subWordSubtrahendHigh, r2
+
+	; Subtract low bytes
+	; a - b = a + (-b) = a + ~b + 1
+	xor r0, r0
+	or r0, r3
+	nand r0, r0
+	load r2, lit01
+	cleq r0, r0, addByte3
+	store subWordResultLow, r0
+
+	; Subtract high bytes with carry-in in r1
+	load r0, subWordMinuendHigh
+	load r2, subWordSubtrahendHigh
+	nand r2, r2
+	cleq r0, r0, addByte3
+
+	; Move carryout to r2
+	xor r2, r2
+	or r2, r1
+
+	load r1, subWordResultLow
+
+	ret
+
+	subWordMinuendHigh: data 0
+	subWordSubtrahendHigh: data 0
+	subWordResultLow: data 0
+
 ; in/out:
 ;  r0:r1 = word
 ; trashes: