One of the more irritating features of working with Microbee stuff is that much of the software is copyright, so I can't distribute it with Freebee unless I get the permission of the copyright holder.
Even worse, much of the recently developed software has been built on disassembled versions of copyright software, so also can't be distributed.
The solution to this is to just go back to first principles and write stuff from scratch. In principle it should be possible to write a CP/M boot ROM and a basic ROM that's compatible but doesn't share the code, in much the same way as with PC clone BIOS ROMs.
This is a big job, but like eating an elephant the trick is to do it one bite at a time and just write code that can be useful. I'm not a programmer, so this is also a learning experience for me.
So let's start with my boot ROM, which until recently just loaded fonts and then dropped into (proprietary) basic. A monitor is a super useful tool in ot's own right for debugging, but as well as that it needs routines for keyboard input, screen output, tape IO, serial IO, etc., which can potentially be repurposed for more ambitious works.
After a week or so of work I have a couple of cool things happening. Firstly a splash screen and boot menu:
And secondly the beginnings of my monitor.
A little more work has a routine that when given an address, displays 32 bytes before and 224 bytes after the address, for a total of 256 bytes. You can move around the memory using the up (back 16 bytes), down (forward 16 bytes), left (back one byte) and right (forward one byte) keys. You can also enter an address using E XXXX where the address is a 4 digit hex number, and the display jumps to that.
I need to generalise input, as the whole checking every keystroke thing is getting bloody tedious. I think a routine to allow entry of a line of text, terminated with a CR character would be just what the doctor ordered. Then I need to parse the text input to determine what to do, and figure out what commands I want the monitor to recognise.
I also need to keep track of what memory is banked in where. There's plenty of space in the bottom right of the screen where I could display current PCG bank, status of boot bit, status or ROMRD bit, status of colouren bit... Then of course I need commands to actually change these, plus (in the case of the boot bit) a way to survive that, as the ROM that's being used for the monitor gets paged out of existance when I deassert boot. Perhaps I could LDIR a short routine to video RAM to do the bank swap, pull the data needed from RAM, and swap back.
Also using PCG bank zero for my scratch and stack isn't so great. Perhaps I could move these to PCG bank 15.
This is seriously good fun though. I'm learning so much about Z80 assembler doing this.
The absence of an up arrow character in the motorola font ROM prompted me to create a new one, including an up arrow plus lots of other stuff from the IBM CGA font ROM, and bold font. It looks so cool. My monitor code now allows me to show the contents of any address, go (call) code at a given address, and has a really advanced edit function where I can edit any memory, move around with arrow keys, and even do relative jumps based on the memory location. And thanks to the bold font it looks really cool.
I've also worked out that my Dell 2001FP accepts the CGA signals without upscaling - all I need is a few resistors to get the levels right. Compare the picture above to previous ones - it's so clear!
I've added code so that I can set the various bank variables (believe it or not there are ten of them!) so that I can show and edit the appropriate bits of memory. The bank variables don't evvect operation of the monitor - it operates in boot mode with pcg 7 most of the time so it has somewhere to store it's stack, but a lot of the time we'll want to view and edit memory that's not shown in these modes. In this case, the edit and display routine will chack where in memory we're working, then set the relevant bank so that we see what we want. Assuming that isn't boot/pcg7, we'll have to be really careful to ensure we don't accidentally pull the code out from under us, so I envisage some copying of code to video ram for execution when we're out of boot mode. For now we're just able to edit the variables using "bank whatever xx".
I've also written a help screen, which can be accessed by typing help. This screen uses as much EPROM as the whole monitor!
