From 7745accfae7621871cb6f5511c9b51f41ae9b25b Mon Sep 17 00:00:00 2001
From: Martin Preuss <martin@aquamaniac.de>
Date: Thu, 5 Sep 2024 18:48:11 +0200
Subject: [PATCH] avr: refactor ctc calculation code.

---
 avr/modules/com2/crc.asm | 43 ++++++++++++++++++++++++++--------------
 1 file changed, 28 insertions(+), 15 deletions(-)

diff --git a/avr/modules/com2/crc.asm b/avr/modules/com2/crc.asm
index 5cd136f..e4afb4c 100644
--- a/avr/modules/com2/crc.asm
+++ b/avr/modules/com2/crc.asm
@@ -14,19 +14,15 @@
 ; add checksum byte to buffer
 ;
 ; IN:
-; - X  : pointer to packet buffer
+; - X  : pointer to packet buffer (points behind checksum upon return)
 ; OUT:
 ; - CFLAG: set if okay, clear otherwise
+; - X  : points behind checksum byte pos upon return
 ; MODIFIED REGS: R16, R17, R18, R19, X
 
 com2CalcAndAddChecksumByte:
-  adiw xh:xl, COM2_MSG_OFFS_MSGLEN
-  ld r18, X                           ; read msg len
-  sbiw xh:xl, COM2_MSG_OFFS_MSGLEN
-  inc r18                             ; account for dest address
-  inc r18                             ; account for msg len byte
-  rcall crc8Calc                      ; (R16, R17, R18, R20, X)
-  st X, r16                           ; add checksum byte
+  rcall com2CalcMsgChecksum
+  st X+, r16                          ; add checksum byte
   sec
   ret
 
@@ -39,15 +35,10 @@ com2CalcAndAddChecksumByte:
 ; - X  : pointer to packet buffer
 ; OUT:
 ; - CFLAG: set if okay, clear otherwise
-; MODIFIED REGS: R16, R17, R18, R19, R20, X
+; MODIFIED REGS: R16, R17 (R18, R20, X)
 
 com2CheckMessageInBuffer:
-  adiw xh:xl, COM2_MSG_OFFS_MSGLEN
-  ld r18, X                           ; read msg len
-  sbiw xh:xl, COM2_MSG_OFFS_MSGLEN
-  inc r18                             ; account for dest address
-  inc r18                             ; account for msg len byte
-  rcall crc8Calc                      ; (R16, R17, R18, R20, X)
+  rcall com2CalcMsgChecksum           ; R16, R18, (R17, R20, X)
   ld r17, X
   cp r16, r17                         ; should be equal
   brne com2CheckMessageInBuffer_error
@@ -58,3 +49,25 @@ com2CheckMessageInBuffer_error:
   ret
 
 
+
+; ---------------------------------------------------------------------------
+; com2CalcMsgChecksum
+;
+; calc checksum from msg buffer (except crc byte)
+;
+; IN:
+; - X  : pointer to packet buffer (points to checksum byte upon return)
+; OUT:
+; - r16: crc8 checksum
+; - X  : points to position of checksum byte
+; MODIFIED REGS: R16, R18, (R17, R20, X)
+
+com2CalcMsgChecksum:
+  adiw xh:xl, COM2_MSG_OFFS_MSGLEN
+  ld r18, X                           ; read msg len
+  sbiw xh:xl, COM2_MSG_OFFS_MSGLEN
+  inc r18                             ; account for dest address
+  inc r18                             ; account for msg len byte
+  rjmp crc8Calc                       ; (R16, R17, R18, R20, X)
+
+