Re: A83: JR command
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Re: A83: JR command
JP is stored with one byte as the opcode (saying that this is a jr command and
nothing else, and what processor flags to care about) followed by one byte
with the offset. This byte can be negative or positive. JP is quicker than JR
because no arithmetic addition has to be done.
When designing processors, there's absolutely no reason to have a whole byte
to specify a bool (i.e. forwards/backwards as you suggested) when a bit is
smaller and easier to read at an electronic level. DD and FD are special
cases, as they have to be distinguishable from all other possible opcodes.
Linus
On 12-Jul-98, Alan C Johnson wrote:
>Just curious, when you JR backwords, does it take up as much space as a
>JP backwords? Here's what I was thinking
>Prgram opocodes:
>$xx; JR command
>$xx; byte telling processor to jump backwards (kind of like DD
>tells it to use IX instead of HL, I think)
>$xx; bytes to jump backwards
>Program opocodes:
>$xx; JP command
>$xxxx; 16-bit address to jump to
>Where all the x's are opocodes I do not know or addresses or whatever.
>I don't know if anyone understands this and it doesn't really matter
>much. JR is faster anyway I think.
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