antirez / c64-hw-simulator

# c64-hw-sim
c64-hw-sim simulates the motherboards of the Commodore 64 (ASSY 250407)
and of the ZX Spectrum 48K (board Issue 3B), pin by pin. You can lift the
leg of a chip, short it to ground or to +5V, pull a chip out of its
socket, or open a resistor. Then you watch what the machine does: the
picture on the monitor, the keyboard, every signal on the board.
The program knows the board's netlist (the electrical connections on the
PCB) and how each chip behaves at its pins. It knows nothing about what a
broken Commodore 64 or Spectrum looks like: when you simulate a broken
part and the screen fills with garbage, that garbage was computed from
the circuit, not chosen by someone who knew what to expect.
Here is an example. Lift pin 17 (A7) of the C64 color RAM before power
on, and the machine boots and looks healthy. The CPU writes and the VIC
reads with the same wrong address, so the RAM simply aliases. Put the leg
back, and the half of the color RAM that was never written shows up as
random colors. Lift it again, and the screen is clean. A real 2114 does
the same.
With this program you can do mainly two things:
1. Learn how the Commodore 64 and the Spectrum work at a more intimate
 level, in a way that is hard to reproduce on real hardware without the
 risk of damaging it (and on most boards many chips are soldered, not
 socketed).
2. If you have a broken C64 or Spectrum and some hypothesis about what
 could be broken, simulate it with this program and perhaps gain clues,
 so that you can desolder chips in the most logical order, and desolder
 as few of them as possible (desoldering is a boring process, and every
 time it risks damaging the board...). A matching symptom is a clue,
 not a proof: different faults can look the same, and the simulation
 has its limits (see below).
Please note that **even on fast computers** this program struggles to
simulate these machines at real speed. A real C64 is simulated at 31% of
its speed on an Apple M3 Max, 39% on an AMD Ryzen AI Max+ PRO 495 (Gorgon
Halo), and 48% on an Apple M5 Max. The ZX Spectrum is simulated at 23%,
28% and 34% of its speed on the same computers. This is not a problem for
the goals of this program, but be warned: it is not an emulator to play
games with.
Why is the Spectrum slower to simulate than the C64, even though its
board is simpler? Because of the time resolution. The ULA is simulated
at a very low level, so that the bus contention just emerges from the
simulation. Its signals are timed to the half period of its 14 MHz
oscillator, so the whole board settles 28 million times per simulated
second, against about 6 million for the C64.
Above: the classic C64 `10 PRINT CHR$(205.5+RND(1));:GOTO 10` maze is
running when the character ROM (U5) is pulled out of its socket, and
later put back. The CPU, the RAM and the program keep working; only the
shapes of the characters are lost, as repairers know well. The scope is
on the data line D0 of the ROM socket: with the socket empty, the bus