[SOLVED] Difference between decrement and subtraction in 68HC12

Difference between decrement and subtraction in 68HC12

I'm using the HC12 chip.

We have instructions SUBA and DECA. The way you use SUBA is by subtracting some value (either in memory or a value you specify) from register A. DECA however takes no parameters and just subtracts $01 from register A. So my question is what is the difference between the instructions SUBA #01 and DECA? My guess was that they behaved differently on negative numbers, but I am not entirely sure. The question I'm dealing with:

      LDAA #230 ; 8 bit system, so this number is technically -26 in 2s complement. Register A is 8 bits, with LDAA loading into register A
LOOP: SUBA #01
      BGT LOOP
; rest of code

Would the code above work differently if SUBA #01 was replaced with DECA?

Solution

DECA is a very common operation and thus the ability to encode it in a smaller instruction was deemed worth its redundancy.