Intel 8080 emulator about cpm65 HOT 2 OPEN

Very nice --- what's the performance like? (I'm guessing, not great...)

Regarding memory access: it'd be nice to have something cross-platform. Traditionally CP/M does extended memory access via a ramdisk. Reading and writing blocks is pretty cheap, although much slower than access via banking, of course. I wonder if some kind of virtual memory would be feasible?

Thinking out loud:

• take the top byte of the address, and with 0xc0, shift right four times.
• this is the offset into an array of 32 pointers in zero page, each of which points at a 2kB local block of memory.
• if the pointer is non-zero, that block is mapped in. Get the pointer, add the address, dereference, return.
• if it is zero, that block's not mapped in, so we follow a slow path to allocate a block and load it from a 64kB swapfile. This may involve evicting another block and writing it back to disk.

That would be something like...

lda vaddr+1
and #0xc0
lsr a
lsr a
tax
lda mappings+1
beq no_mapping
clc
lda vaddr+0
adc mappings+0, x
sta paddr+0
lda vaddr+1
and #0x3f
adc mappings+1, x
sta paddr+1
<derefence paddr here>

That's.. not that bad? It's more work than your current code but it's portable. Coming up with a cache eviction strategy would be tricky. LRU would require overhead for every memory access, but apparently some ARM chips would just evict a block at random and that was considered good enough...

from cpm65.

I'm afraid that will slow things down by at least a factor of two, not counting the load and save times to ramdisk. Most optimizations that I use can't be used, like the msb_to_bank and msb_to_adjusted tables, and checking for end of bank with the BIT instruction. Checks and adjustments that have to be done for almost each memory read or write (end-of-bank only during the optimized instruction fetcher).

But I see your point. It would be nice to have a cross-platform 8080.COM but IIUIC that would offload the bankswitching to a platform dependent ramdisk driver. IMHO it's better to keep the platform dependent code as close to the emulation as possible. It's slow enough as it is. Although it's faster than I had expected, I do not have a number yet. I need a proper benchmark program that I can run on a 2MHz or 4MHz 8080 and on the emulator to see how it performs. Do you know about such program?

I think I'll keep the bankswitching in for now. Later, if there's multiple drives and ramdisk support we can always change it or make it optional.

Regards,
Ivo

from cpm65.