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Old 28th May 2019, 4:55 pm   #8
TonyDuell
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Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 3,217
Default Re: Fun with 6502 Assembler

Not quite.

The point being that 'NOP' as an istruction has a specific binary value.So NOP NOP specifies both bytes uniquely. And NOP NOP NOP specifies all 3 bytes. But for BRN, the first byte has a specific value (to make it a BRN) but the second byte can be anything. And for a LBRN the second and third bytes can be anything.

That's what makes the trick I mentioned work. You have an LBRN with the following bytes (which would be the displacement if the long branch ever happened) the right pair of bytes for an LDA (immediate) instruction and its operand. If you execute them starting with the 'LBRN' byte then they are ignored (the processor takes them as a displacement for a branch which never occurs). But if you jump to the byte after the LBRN (that is to the first byte of the 'displacement') then of course they are used as an LDA instruction.
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