Monday, March 03, 2025
A quirk of the Motorola 6809 assemblers
I just learned an interesting bit of trivia about 6809 assembly language on a Discord server today.
When Motorola designed the 6809 assembler,
they made a distinction between the use of n,PC and n,PCR in the indexing mode.
Both of those make a reference based off the PC register,
but in assembly language they defined,
using n,PC means use the literal value of n as the distance,
whereas n,PCR means generate the distance between n and the current value of the PC register.
I never knew that.
I just looked and all the materials I had on the 6809 use the n,PCR method everywhere,
yet when I wrote my assembler,
I only support n,PC and it always calculates the distance.
I think I forgot that it should have been n,PCR because on the 68000
(which I also programmed,
and was also made by Motorola) it always used n,PC.
And I don't think I'll change my assembler as there does exist a method to use an arbitrary value of n as a distance:
LDA (*+3)+n,PC.
The asterisk evaluates to the address of the current instruction,
and by adding 3 you get the address of the next instruction,
which in the PC-relative addressing mode,
is a distance of 0.
Then n will be the actual offset used in the instruction.
Yes,
it's a bit convoluted,
but it's a way to get how Motorola originally defined n,PC.
And apparently, Motorola defined it that way to make up for less intelligent assemblers back in the day due to memory constraints. We are long past those days.
