Author Archives: Eric

Since I’m basically done with my original project, the rewrite of the Apple 1 monitor for the MC6800, I’m now spending my copious free time working on HP-41 stuff. I’m trying to hack together an expansion ROM that will support … Continue reading →

A short (1:45) video demo showing my rewrite of the Apple 1 monitor for the MC6800 microprocessor, running on a modified MESS simulator: http://www.youtube.com/watch?v=Nh7qqymZeiQ I hope to have in the near future a video of the rewritten monitor running on … Continue reading →

I wrote a trivial 6800 program to test my rewritten monitor. It just outputs the character set to the display continuously. org $0300 test: ldaa #$a0 loop: jsr echo inca cmpa #$e0 bne loop bra test Enter it with this … Continue reading →

After further debugging, my monitor for the Apple 1 with MC6800 microprocessor now works, at least on the MESS simulator. I’ve used it to examine and modify memory, and to run a small test program in RAM. The monitor still … Continue reading →

I fixed some bugs in both my splitroms.py utility and my MAME patch. Now I’ve got my MC6800 monitor for the Apple 1 actually running well enough to enter command lines, though it doesn’t execute them properly. It does automatically … Continue reading →

The Apple 1 uses four-bit-wide bipolar PROMs, so MESS uses ROM image files containing only the low nibble or only the high nibble of the code, stored one nibble per file byte.  I’ve written a splitroms.py Python script to take … Continue reading →

I’ve got a patch to MESS to add simulation of an Apple 1 with an MC6800 processor:  http://pastebin.com/tN3DXX7u The system name is “apple1_mc6800″. The ROM is not included in that patch, and the ROM names, CRCs, and SHA1SUMs are not … Continue reading →

I’ve got the Apple 1 monitor for the MC6800 processor nearly completely written, but haven’t done any testing yet.  I haven’t posted more to the blog because there hasn’t been much interesting to say. At this point it looks like … Continue reading →

An interesting “feature” of the Apple 1 monitor is that it does not initialize the stack pointer. In the MOS Technology MCS6502 microprocessor, the most significant eight bits of the stack pointer are hardwired to $01. That means that the … Continue reading →

My Retrochallenge 2011 project is to rewrite the Apple 1 monitor for use on the Motorola MC6800 microprocessor.  Steve Wozniak primarily intended the Apple 1 to use the MOS Technology MCS6502 microprocessor, also used in the later Apple II and … Continue reading →