Electronics for Newbies: A working copy of CP/M

Last time I left off, I had a broken copy of CP/M. I've now whacked the basic bugs on the head, and it can run some simple executables. For some reason, MBASIC isn't working, but I'll leave that for another day.

There were various minor odds and ends, but in the end it came down to two problems, both embarassing in retrospect. The first was that the whole thing was pretty unstable. I could run a "dir" command, and it would work, more-or-less (see below), but then crash the second time I tried the command. It smelled like memory corruption - but what could be going on? The BIOS code I wrote was really very simple and I saw no reason for it to be doing bad things.

Eventually, I worked out it was the memory mapping. The same page was mapped in twice, so modifying it in one mapping corrupted it in the other. This in turn happened because I configured the mapping of the final page (mapping over where the EEPROM image goes, which I only do once we're done with the monitor) in two places - originally, I set up the mapping in CP/M initialisation code, but then I moved it to the boot loader... however, the bit of code left in CP/M got out of date, and clashed with the mapping for the rest of memory. Ooops. I removed the copy in the CP/M initialisation code, and the problem went away.

However, all was not well. "Dir" only ever returned a single entry, and "type" read the file but then gave up after a single character. I started digging into the source of "dir" in order to understand the problem... and it was very, very silly! Both "dir" and "type" stop if you press a key. My terminal emulator was set up to send "\r\n" at the end of a line, but CP/M only needs the one. It treats the other character as another keypress, and immediately gives up. I changed my terminal emulator settings, and suddenly everything was good:

CP/M-80 Version 2.2c For the Dirac SBC

A: MORE     COM : LOAD     COM : RW13     COM : APDOS    COM
A: BHEAD    C   : CPM56    COM

There's an awful lot left that's basically not right yet, but I finally feel that the basics are there, and this is in some sense a working CP/M computer.

Posted 2016-07-16.