This is a 65C02 virtual machine for my V-M8 microcomputer (ATmega162 with 64KB of external ram). It has only be tested by running two roms: Microsoft's BASIC-65 (as found on KIM-1, Pet Commodore, Tangerine's Microtan...), and its derived version found on the Oric Atmos (Tangerine Basic 1.1). The emulated 65C02 runs at approximatively 4 MHz on a 16 MHz ATmega162. I don't intend to put additional work on this virtual machine, because other virtual machines are more useful (i.e give access to a wider library of software, such as the Z80+CP/M virtual machine) or are more interesting (such as the Turbo Modula-2 Reloaded virtual machine).
"Hybrid65C02" emulator by F.Frances, (C) 2019.
Emulates a 65C02 :
- all instructions of the 65C02
- additional COP instruction that calls AVR routines
The aim is to provide an 8-bit 65x02-like processor with access to 64 KB of SRAM space, so this targets ATmega processors with external SRAM (up to 64 KB) such as ATmega161/162/640/1280/2560/1281/2561...
Caveats:
- Decimal mode is not implemented
- the stack is shared between the emulated 65x02 cpu and the host AVR.
The emulator does not use the stack except when calling AVR routines
from within the COP instruction, so actually the AVR stack starts from
the top of the emulated 65C02 stack => care must be taken that the
cumulated stack size doesn't exceed page 1.
- interrupts may be serviced *in the middle* of the emulation of a
single 65x02 instruction, so caution must be taken when reading/writing
volatile data in the 65x02 code (data that is also read/written in the
AVR interrupt handlers) => use the TSB and TRB to solve synchronization/
communication issues between the 65x02 emulated code and the AVR handlers
or protect critical sections with cli/sei.
- indirect modes must not use pointer in direct page's location $FF
- page 0 is translated to another page. This is mandatory as the ATmega
uses page 0 for its registers and IOs.
- RTI and RET don't check if return address is in zero page
Speed:
Running the following program within BASIC-65 takes 10 seconds on a 16 MHz
ATmega162, while the same program takes 40 seconds on a PET Commodore,
hence a 16 MHz ATmega162 emulates a 4 MHz 6502...
10 FORI=1TO10000
20 A=A+I
30 NEXT
ATmega162 timing examples for some 6502 instructions:
NOP 6 cycles
INX 8 cycles
PHA 8 cycles
PLA 10 cycles
LDA #imm 10 cycles
LDA direct 12 cycles
LDA direct,X 13 cycles
LDA absolute 18 cycles
LDA absolute,X 19 cycles
LDA (indirect) 20 cycles
LDA (indirect,X) 21 cycles
LDA (indirect),Y 21 cycles
BEQ relative 11/14 cycles
JMP absolute 15 cycles
STA direct 10 cycles
ADC direct 16 cycles
ASL absolute 21 cycles