6805 branch and compare instruction conditions/results

[Scratch.HTF]

I am in the process of converting code from a Motorola 6805 series (MC68705P3) microcontroller to another architecture, but I am having trouble identifying what conditions (or what registers) are compared in the Branch and Compare instructions (also for Compare, where the result is stored):

eor   #$20   // Exclusive OR Memory with A
bne   L013C   // Branch if Not Equal

lda   X0014   // Load A from Memory
bne   L015C   // Branch if Not Equal

cmp   #$01   // Arithmetric Compare A with Memory
beq   L01AE   // Branch if Equal

[amyk]

The questions you ask suggest that you haven't familiarised yourself with the architecture yet; give the first half of this a quick read first:

http://bitsavers.trailing-edge.com/components/motorola/6805/6805_Users_Manual_2ed_1983.pdf

Then use the instruction set reference in the second half for the details on what each of the instructions do.

[westfw]

The branches are conditional based on bits in the “condition code register” that were set as the result of some previous ALU operation (xor, add, sub, etc)


The CMP instruction works like subtract, but does not store the result.  It ONLY sets bits in the CC register.

[thinkfat]

bne/eq branches based on the 'zero' flag. The zero flag is set in the result of an operation is 0.

Beq branches if the zero flag is set, bne not.



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