I've been halfway writing my own simulator to assist in documenting some old 6502 code but see the Kowalski simulator is a popular package here. Unfortunately all the posted links to it are broken.
Anyone know a way / link to get a copy of it?
Welcome to the forum.
The Kowalski simulator is an editor, macro assembler and MPU simulator all rolled into one Windows package. I've extensively used it to test code concepts and, in fact, wrote the entire BIOS ROM for my POC computer
with the simulator. The sim has a few bugs, which are mostly nuisances. However, one bug to watch out for occurs when executing as an NMOS 6502 and processing BCD arithmetic. A sequence such as the following:
will improperly set the .Z flag if run in 6502 mode.
The sim can be set up to recognize 6502 or 65C02 instructions with a pseudo-op as the first code line in your source, e.g.:
which tells the assembler that the 65C02 instructions (e.g., PHX) are valid and that unquoted text is not case sensitive (a recommended setting, given that traditional 65xx assembly language is case-insensitive).
The package simulates the Rockwell model, which means it isn't 100 percent compatible with the WDC 65C02. I have generated a longish macro file which adds WDC 65C02 instruction support, as well as support for most of the W65C816S native mode features. The source code for the simulator is also available somewhere, but was written using Microsoft development tools and is commented in Polish, making it all but impossible to modify or upgrade.