From 108bf334e8de3853539d344e093b68798e2cf6a4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Juhani=20Krekel=C3=A4?= <juhani@krekelä.fi>
Date: Thu, 15 Sep 2022 17:41:44 +0300
Subject: [PATCH] Implement bcdDigitAdd3 in calculator.asm

---
 calculator.asm | 58 ++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 44 insertions(+), 14 deletions(-)

diff --git a/calculator.asm b/calculator.asm
index 7224eb8..1073b97 100644
--- a/calculator.asm
+++ b/calculator.asm
@@ -438,8 +438,8 @@ fResult:
 ; Arithmetic
 ; ------------------------------------------------------------------
 
-; x y -- tens(x+y) ones(x+y)
-bcdDigitAdd:
+; x y z -- tens(x+y+z) ones(x+y+z)
+bcdDigitAdd3:
 	cleq r0, r0, stAdd
 
 	cleq r0, r0, stDup
@@ -447,18 +447,43 @@ bcdDigitAdd:
 	load r1, #a
 	cleq r0, r0, pushWord
 	cleq r0, r0, stGtEq
+	breq r0, r1, bcdDigitAdd
+
+	; Adjust so that carry is in the high 3 bits
+	; Basically we want 9+1=a₁₆ to become 20₁₆
+	xor r0, r0
+	load r1, #16
+	cleq r0, r0, pushWord
+	cleq r0, r0, stAdd
+
+; x y -- tens(x+y) ones(x+y)
+bcdDigitAdd:
+	cleq r0, r0, stAdd
+
+	cleq r0, r0, stDup
+	; Only look at the bottom 5 bits, since high 3 bits may already
+	; contain carry
+	cleq r0, r0, popWord
+	load r2, #1f
+	and r1, r2
+	cleq r0, r0, pushWord
+	xor r0, r0
+	load r1, #a
+	cleq r0, r0, pushWord
+	cleq r0, r0, stGtEq
 	breq r0, r1, bcdDigitAddNoAdjust
 
-	; Adjust so that carry is in the high nybble
-	; Basically we want 9+1=a₁₆ to become 10₁₆
+	; Adjust so that carry is in the high 3 bits
+	; Basically we want 9+1=a₁₆ to become 20₁₆
 	xor r0, r0
-	load r1, #6
+	load r1, #16
 	cleq r0, r0, pushWord
 	cleq r0, r0, stAdd
 
 bcdDigitAddNoAdjust:
 	cleq r0, r0, stDup
 	cleq r0, r0, stShr4
+	cleq r0, r0, stShr1
 	cleq r0, r0, stSwap
 	breq r0, r0, stLowNybble
 
@@ -618,11 +643,7 @@ fPrint:
 		; added to the corresponding minuend. This by itself gives a
 		; result that is one too small, so we add one to it (c.f. binary
 		; subtraction, where we negate the subtrahend and then add one).
-		; Normally we'd have to split subtraction into two rounds, one
-		; to add the complemented subtrahend and one to add the 1, but
-		; since our minuend is a constant, we can add 1 to it instead.
-		;    501     + 999 - biased
-		;  = 500 + 1 + 999 - biased
+		;    500 + 1 + 999 - biased
 		;  = 500 + 1000    - biased   (1000 = 0 mod 1000)
 		;  = 500           - biased
 
@@ -649,14 +670,11 @@ fPrint:
 		xor r0, r0
 		load r1, #5
 		cleq r0, r0, pushWord
-		cleq r0, r0, bcdDigitAdd
-		cleq r0, r0, stSwap
-		cleq r0, r0, popWord
 		xor r0, r0
 		load r1, fArg1+1
 		cleq r0, r0, pushWord
 		cleq r0, r0, bcdDigit9sComplement
-		cleq r0, r0, bcdDigitAdd
+		cleq r0, r0, bcdDigitAdd3
 		cleq r0, r0, stSwap
 		cleq r0, r0, popWord
 
@@ -779,6 +797,18 @@ stShr4:
 	shr r0, 4
 	breq r0, r0, pushWord
 
+; a -- a>>1
+stShr1:
+	cleq r0, r0, popWord
+	shr r1, 1
+	xor r2, r2
+	or r2, r0
+	shl r2, 4
+	shl r2, 3
+	or r1, r2
+	shr r0, 1
+	breq r0, r0, pushWord
+
 ; a -- a&f
 stLowNybble:
 	cleq r0, r0, popWord