Saturday, April 1, 2017

Emulating an 8080 on the MEGA65

I must say I was very impressed to discover what LGB has been up to lately:

As the URL suggests, he has been working towards CP/M emulation on the MEGA65.

What is remarkable is that he is currently obtaining the performance of an approximately 12MHz 8080 CPU on the MEGA65, i.e., about 3x faster than many of the real CP/M computers -- but this is emulation using the 45GS10 CPU of the MEGA65 to achieve this.  Put another way: The 45GS10 can emulate an 8080 at fully 1/3 of the clock speed of the 45GS10.  

Both he and I were at first rather surprised that it could emulate at such speed.  However, on inspection, it turns out that the 45GS10 is quite friendly for emulating foreign hardware.  Specifically, the combination of a large address space, together with the ZP-indirect 32-bit addressing mode and the JMP ($nnnn,X) jump-table instruction means that an emulator can operate in its own address space, separate from the emulated system, and use ZP registers as emulated registers, with the ZP-indirect 32-bit mode allowing dereferencing of those emulated registers.