HP-41 printer interface question. (To the M-code gurus out there)


Hi all,

I've read almost every line of info regarding HP-41 <> PRINTER interface.

Still a (great) doubt remaining; quoting William Doug Wilder:

"It makes me wonder how the carry
bit gets set, what is the hardware sequence that sets carry?"

So far I've been unable to figure out how instructions h'003, h'043 & h'083 handle the carry flag *into* NUT CPU.

These are instructions decoded and executed by the Peripheral chip, issued right after the SELP 9 instruction.

Any comment will be much appreciated.

Thanks for your attention



For the flag tests of the "intelligent peripheral" chips, the peripheral puts the flag value onto the ISA line during bit time 0. The CPU copies that to the carry flag.

This is for any instruction of the form hexadecimal x03, x43, x83, or xc3 (x being 0, 1, 2, or 3), in a sequence following a peripheral select instruction (x24, x64,xa4, xe4). It does not apply to the "normal" x03, x43, x83, xc3 instructions, which are jump instructions.


Hi Eric,

Thanks a lot for your reply... :-) now back to work!



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