Be careful Lee,? ?all that is going to go straight to my head!
The single step is definitely cool, but as I recall, it was your idea, I just coded it.
For those that don't know it.? Many single step operations write a jump to the next instruction(s) possible to be executed.? This jump returns control to the monitor program.? Ok, it might not exactly be a jump, some use Return, most use the Restart instructions, but it's all the same mechanics.
Lee suggested that the jump be externally generated though the timer interrupt.? In this fashion, it simplifies the need to write a jump into RAM and also permits single stepping through ROM (where you can't temporarily write a jump).? This single step code is in fact within the ISR and it simply wastes exactly the number of cycles so that when the ISR returns to main line code, only 1 instruction can execute before an interrupt brings the CPU back into the ISR.? That's the basic principle, but there's just a little more, a gory detail.??
Gory Details.
The Z80 has variable length instructions, taking between 4 and 17 cycles more or less (check data sheet for exact numbers).? The interrupt works from a counter after 4096 counts I think (that's about 1mSec with a 4Mhz clock).? In order to perfectly align the interrupt for a single instruction, a halt instruction is used to wait for 1 timer interrupt interval before counting out the exact count of cycles.
Cheers,
Josh
