Kill the bit program does not work as expected

[Selorax]

The program itself is a classic game for Altair. https://altairclone.com/downloads/killbits.pdf

Steps to reproduce:

1. Open https://wixette.github.io/8800-simulator/ in browser
2. Switch the machine ON
3. Switch to Debug tab
4. Input the following hex string:

21 00 00 16 80 01 0E 00 1A 1A 1A 1A 09 D2 08 00 DB FF AA 0F 57 C3 08 00

5. Switch back to Simulator, click Reset and Run

Expected result: the top 8 address LEDs show a moving light while top address switches invert its state

Observed result: neither top 8 address LEDs nor data LEDs a lit while the program is executed. Switches seem to do nothing.

The program shows the same behavior when input by switches manually as well.

[t0maszduda]

Yes, for me too :(

[XujieSi]

An easy hack is to fix the LED display after each step(): https://github.com/wixette/8800-simulator/blob/master/js/sim8800.js#L380 replace cpu.pc with cpu.pc | cpu.d * 256.

Basically we always show the address stored in the register d. Kill-the-bit program leverages a side effect of Altair 8800 -- displaying that address stored in d when executing the assembly instructionldax d (corresponds to 1A). However, the current simulator does not capture this side effect. To be more systematic, a careful check about whether current assemble instruction is indeed ldax d shall be made, while the hack above simply ignores it.