On 12/24/25 15:01, Robert Armstrong wrote:
Tom Uban <[email protected]> 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
You can find lots of useful info here:
https://www.jkearney.com/Tiny8demo/Datasheets/8008UM.pdf
https://bytecollector.com/archive/mark_8/My_Mark-8_Info/Software/8008_InstructionSet_CodeSort.PDF
You are correct that there has to be a temporary register pair (I use DE).
On my board, the only interrupt is an 8251 serial controller, which has an internal interrupt
enable/disable.
intsav: db 0,0,0,0,0,0 ;interrupt save regs A, flags, B, C, H, L
warm:
lhli (intsav+5) ;point to RAM location used for saving L register
lem ;load E register with saved L
dcl ;point to H register save location
ldm ;load D register with saved H
dcl ;point to C register save location
lcm ;restore C register
dcl ;point to B register save location
lbm ;restore B register
dcl ;point to flags byte
lam ;pickup flag information
ada ;restore all flags
ret ;return from interrupt
--tom
