I notice that the 0x00-ff memory space has no 0xff byte and that 0xfe seems to change a lot, but it's not explained. What's going on here? What's in 0xfe and why is 0xff not shown?
I noticed that there's no multiplication operator. Does that mean that multiplication was performed using looping addition? What about division, floating-point, et al?
* This is an old processor, so it doesn't support fancy stuff like multiplication and division and floating-point. Generally you just try to avoid multiplication whenever possible, or keep it to powers of 2, since that can be done with shift operations. You can do multiplication using looped addition, but if you were to implement it you'd want to use a constant-time binary multiplication algorithm instead.
Moreover the content of 0xFE keeps changing even after execution stops (if you keep hitting the "step" button).
0xFF isn't even listed, but a load from that address seems to work (with value 0x00). [ETA: actually, that seems to be a quirk of the Monitor's initial config. If you set "length" to 100 instead of ff, you can see byte 0xFF in the monitor. Nothing wrong or weird there.]
0xFF doesn't show up because the Monitor's length is set to 0xFF, which is 255. If you set the length to 0x100 it properly shows the first 256 entries.
As for the value at 0xFE, I don't know.
There is a function called 'setRandomByte' that is called on every debugger step, but it has no comments so it may just be leftover debugging logic.
0xfe is a random number - a new random byte is generated on every instruction.
0xff contains the ASCII code of the last key pressed.
My simulator is adapted from the one at http://6502asm.com/beta/index.html - that has a help screen with some more info. I mostly cleaned up the (atrocious) JavaScript and added a memory monitor and disassembler (and implemented a few instructions they'd left out).