[cctalk] Re: i8008 and interrupts

On Dec 24, 2025, at 1:44 PM, smbaker(a)gmail.com wrote: When I did this, I remember there was a pair of registers that I could not save, probably de. I can look and see if I ended up with useful isr code in my project. Maybe the real Intended use is to be able to halt and resume on interrupt. There was a way to cleverly restore the flags. My isr code is in https://github.com/sbelectronics/h8/blob/master/h8-8008/asm/fpanisr.inc Sent from my iPhone >> On Dec 24, 2025, at 1:02 PM, Robert Armstrong via cctalk <cctalk(a)classiccmp.org> wrote: >> >  >> >> Tom Uban <uban(a)ubanproductions.com> wrote: >> Here is the power-up/interrupt code I use on a board I built: ... > > Thanks! A couple of questions - > > It looks like this code trashes DE, so I assume the background code just can't use those registers? So there's no way to code a truly transparent ISR? > > I also notice that the 8008 lacks any kind of interrupt enable F-F, nor any EI/DI type instructions. Is that right? I guess you could always implement those with external hardware, at least. > > And the big question - what's your code for restoring these flags when you exit the ISR? The MCS-8 manual says that all ALU instructions always update all flags, with the exception of the rotate opcodes which only change carry. It seems that means the ZS&P flags at least would have to be restored together in one instruction. > > Bob >