# [virtmach] instruction set

**Dennis Yelle**
dennis51@jps.net

*Tue, 18 Apr 2000 12:11:06 -0700*

Dennis Yelle wrote:
>*
*>* thaddaeus.frogley@creaturelabs.com wrote:
*>* >
*>* > Hi,
*>* >
*>* > Quick question. Don't feel obliged to justify your answer.
*>* >
*>* > When trying to create an instruction set for a 3 address machine, assuming
*>* > that the following already exist:
*>* >
*>* > jump target
*>* > jump-if-zero a,target
*>* > jump-if-non-zero a,target
*>*
*>* Well, assuming I already have:
*>*
*>* subtract a,b,c
*>* jump-if-negative a, target
*>*
*>* And assuming I am short of op code space,
*>* I don't need any of these:
*>*
*>* > jump-if-equal a,b,target
*>* > jump-if-less a,b,target
*>* > test-if-equal a,b,result
*>* > test-if-less a,b,result
*
Well, now that I think about that for a few more minutes,
I see that I was wrong about that.
Because this:
if ( a < b )
is not the same as this:
if ( (a-b) < 0 )
Sorry.
Dennis Yelle