Memotech Bill wrote: ↑
Fri Aug 27, 2021 4:42 pm
The Pico has a total of 264KB of RAM. Any RAM used for video will come out of that available for Basic.
Lots of good memories of using the Beeb from this thread -- a Beeb was my second computer. At the time, I worked for a BBC Micro reseller. I'm sure the primary participants know most of what follows, but here are some details for the rest of us. I still have access to the User Guide (including Preliminary and Advanced versions).
The BBC Model B had 32KB RAM. Any RAM used for video came out of that available for Basic. This made the highest resolution modes virtually useless. Modes 0-3 used too much memory for use on the 16K RAM in a Model A.
Modes 0-2 used 20KB RAM. Only a few K were available for programs and variables in these modes, around 8KB depending on the hardware fitted.
The Mode 0 resolution was 80x32 characters or 640x256 graphics, in two colours. Modes 1 and 2 traded more colours for lower horizontal resolution. Mode 3 was 80x25 text only, but still used a 16KB bitmap. Modes 4-6 were half resolution versions of modes 0, 1 and 3. Mode 7 (Teletext) provided 40x25 characters in 1KB of RAM, block graphics only, in 8 colours. IIRC the control characters (colour and text/graphics, flash, etc) occupied a character position.
Admittedly, the Beeb then had a graphics processor (6845) and a ULA to generate the video signal, but 256K RAM would have been an unthinkable luxury! None of it was VGA compatible -- VGA was not introduced until 1987. Finally, the UK version as described here generated a PAL compatible signal (available as RGB, composite, or UHF modulated). The American version used fewer rows in each mode to give an NTSC-compatible signal.
I understand that the current project is about getting BBC BASIC running on a Pico -- however, it would be interesting to see how much of a BBC Microcomputer could be emulated on Pico-based hardware.