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TonyDuell · 28th May 2019, 1:09 pm

A trick I remember on the 6809 (beautiful processor!) if you wanted to run the same routine with one of two values in the A register.

The 6809 had 'long branch' instructions which were 3 bytes long and allowed you to branch to anywhere in memory (not just within +/-127 bytes of where you are as with normal branches). It also had a complete set of conditional branch (and long branch) instructions, including branch always (BRA and LBRA) and branch never (BRN and LBRN). The former would do the branch (effectively a relative jump) no matter what the flags were, the latter would never branch.

So you started the routine with a LDA instuction to load the A register with one of the 2 values. Then a LBRN with a 16 bit offset that happend to be an LDA for the other of the 2 values. Then continue the routine.

So if you jumped to the first LDA, the accumlator was loaded with the first value. The LBRN did nothing, and then the routine continues. But if you jumped to one byte after the LBRN opcode, the offest was taken as an LDA instruction with the second value, And of course it was followed by the rest of the routine.

28th May 2019, 1:09 pm	#4
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,213	Re: Fun with 6502 Assembler A trick I remember on the 6809 (beautiful processor!) if you wanted to run the same routine with one of two values in the A register. The 6809 had 'long branch' instructions which were 3 bytes long and allowed you to branch to anywhere in memory (not just within +/-127 bytes of where you are as with normal branches). It also had a complete set of conditional branch (and long branch) instructions, including branch always (BRA and LBRA) and branch never (BRN and LBRN). The former would do the branch (effectively a relative jump) no matter what the flags were, the latter would never branch. So you started the routine with a LDA instuction to load the A register with one of the 2 values. Then a LBRN with a 16 bit offset that happend to be an LDA for the other of the 2 values. Then continue the routine. So if you jumped to the first LDA, the accumlator was loaded with the first value. The LBRN did nothing, and then the routine continues. But if you jumped to one byte after the LBRN opcode, the offest was taken as an LDA instruction with the second value, And of course it was followed by the rest of the routine.