Re: BRK opcode

From: Brian Ketterling (
Date: 2003-11-06 01:18:57

Just 2 cents...

>From: Steve Judd <>
>Yes, it is actually a two-byte opcode -- the PC counter goes to PC+2.
>But nothing is actually done with the second byte; if it were e.g. pushed
>onto the stack it might have been useful as a software interrupt...
>...Sorry to follow-up on my own message, but I forgot to mention that the 
>65816 data sheets call out BRK as two bytes -- "officially", it's
>documented as a 2-byte opcode.

I'm away from my 65816 docs, so I'm not sure, but I *think* on that MPU it 
is possible to pass the second byte as a parameter.

>From: "Gideon Zweijtzer" <>
>Of course, only the design team of the 6502 knows for sure what the
>reason is...

I think the design team was one or maybe two guys, working primarily on 

>...The size of a 6502
>instruction is easily derived from the opcode by the following formula:..

That's pretty clever!

-- Brian

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