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dominicbeesley · 3rd Aug 2020, 10:42 am

I suppose I'm more concerned with how it does it rather than just the effect the "I" instructions on the datasheet (with opcodes of the form "11xxx100") work differently (and have fewer cycles - an artifact a programmer will be interested in!) than the others.

Another aspect that would be useful to a programmer would be for an STI the second opcode byte would possibly be ignored in terms of forming the address.

consider the "C8" case:

[code]
LDI 04
ST 0(PC)
[code]

If this were executed twice from RAM on the second pass it would actually do a ST 4(PC)...possibly not what was wanted though with some byzantine thinking it might be possible to make use of it?

If CC works then the immediate value might be overwritten but ignored in forming the address so I think the distinction is very much valid.

3rd Aug 2020, 10:42 am	#110
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Re: SC/MP timing and microcode information I suppose I'm more concerned with how it does it rather than just the effect the "I" instructions on the datasheet (with opcodes of the form "11xxx100") work differently (and have fewer cycles - an artifact a programmer will be interested in!) than the others. Another aspect that would be useful to a programmer would be for an STI the second opcode byte would possibly be ignored in terms of forming the address. consider the "C8" case: [code] LDI 04 ST 0(PC) [code] If this were executed twice from RAM on the second pass it would actually do a ST 4(PC)...possibly not what was wanted though with some byzantine thinking it might be possible to make use of it? If CC works then the immediate value might be overwritten but ignored in forming the address so I think the distinction is very much valid.