jeffjetton/pysim6502

Python-based 6502 microprocessor simulator

PySim6502

PySim6502 simulates a simple computer system based around the classic MOS 6502 microprocessor. It's particularly handy for iteratively developing and testing assembled 6502 code without having to transfer it over to actual hardware each time.

• Written entirely in Python for ease of modification and customization
• Aims for fairly loose coupling between components. A new system can be quickly created as a subclass of the Bus object, overriding just what is necessary to implement a particular memory map.
• Commands are similar to simh:
  • Examine and deposit values into memory
  • View status of CPU registers
  • Single-step instructions
  • Set breakpoints

Type help at the sim> prompt to learn more.

Example of Use

Run the sim.py file to start. Try entering bus to view details of the Bus object currently being used by the CPU:

python sim.py
sim> bus
Apple 1 Bus
  0000-7FFF: Read/Write
  8000-DFFF: Not Used
  E000-FFFF: Read-Only
Output Delay: 5000μs
Memory Errors: ON
Addresses D010-D013 used for input/output
sim> 

As you can see, by default the simulator is set up to emulate an Apple 1 with 32K of RAM and 8K of ROM.

The WOZMON monitor and Integer BASIC are automatically written to ROM, although you can change this in the SystemBus constructor found in apple1_bus.py.

Examine the first few bytes of Integer BASIC, which starts at $E000:

sim> examine e000 e01f
E000: 4C B0 E2 AD 11 D0 10 FB
E008: AD 10 D0 60 8A 29 20 F0
E010: 23 A9 A0 85 E4 4C C9 E3
E018: A9 20 C5 24 B0 0C A9 8D
sim>

Input is case-insensitive, but hexadecimal is assumed for address arguments (do not prefix with $ or 0x, etc.)

Enter start to run the simulated system. You'll immediately drop into WOZMON. If you examine the same address range viewed previously (using WOZMON's "dot" syntax) you should see the same values:

E000.E01F

E000: 4C B0 E2 AD 11 D0 10 FB
E008: AD 10 D0 60 8A 29 20 F0
E010: 23 A9 A0 85 E4 4C C9 E3
E018: A9 20 C5 24 B0 0C A9 8D

The difference is that now the simulated monitor is doing the examining rather than the simulator itself!

Start BASIC by entering its starting address (E000) followed by R. You should eventually be greeted by the > prompt:

E000R

E000: 4C
>

Enter the first line of our test program:

>10 FOR I = 1 TO 5
>

Now type Control + E to exit back to the simulator prompt. The simulated system will be frozen in place where you left it, and you can enter cpu to view register and system status at that point:

>cpu
A: 80   X: FF   Y: 00    NV_BDIZC   HALTED
S: FB  PC: E006          10000001   Delay: 1μs
sim>

Integer BASIC's input buffer, which holds everything you type in as you enter it, starts at $0200. If you examine memory there, you should see the remnants of the line you just wrote. Although it will be in Apple-1-style ASCII values, which have bit 7 always set:

sim>e 200 210
0200: B1 B0 A0 C6 CF D2 A0 C9
0208: A0 BD A0 B1 A0 D4 CF A0
0210: B5
sim> 

Note that you can abbreviate examine to exam or just e.

Enter start (or run or just r) to pick up the simulation where you left off, then type in the remaining lines of the program:

>20 PRINT "HELLO!", I
>30 NEXT I
>40 END
>LIST
   10 FOR I=1 TO 5
   20 PRINT "HELLO!",I
   30 NEXT I
   40 END 

>RUN
HELLO!  1
HELLO!  2
HELLO!  3
HELLO!  4
HELLO!  5

Control + R performs a simulated 6502 reset, which will drop you out of BASIC and back into WOZMON.

Checking for Ctrl+R and Ctrl+E currently only happens when the simulated system is looking for keyboard input. In the case of an endless loop, you'll usually have to resort to quitting Python entirely with Ctrl+C.