The memory map on p500 is correct - for an UNEXPANDED BBC Micro
But as you add additional Paged ROMS, they may request memory from the MOS, in which case OSHWM moves upwards to accomodate them.
Acorn's DFS (Disc Filing System) requests 11 pages of RAM for its own use (11 x 256 bytes = 2.75 Kb), so OSWHM rises from &E00 to &1900 to accommodate this request. DFS is free* to use the memory between &E00 and &18FF for its own purposes. *actually, there's public and private RAM allocated, so it's a bit more complex, but you get the idea.
By default, BASIC will set PAGE to OSHWM (OS High Water Mark - see p431). But you can move PAGE up and down to you heart's content. Remember, BASIC and the MOS are independent of each other!
Now, for added confusion, on a Master that memory used by Paged ROMs is hidden away elsewhere by the MOS, so OSHWM will generally stay around &E00.
And if you're running on a 6502 second/co processor, I believe OSHWM is at &800.
OSHWM is a moveable feast!
But as you add additional Paged ROMS, they may request memory from the MOS, in which case OSHWM moves upwards to accomodate them.
Acorn's DFS (Disc Filing System) requests 11 pages of RAM for its own use (11 x 256 bytes = 2.75 Kb), so OSWHM rises from &E00 to &1900 to accommodate this request. DFS is free* to use the memory between &E00 and &18FF for its own purposes. *actually, there's public and private RAM allocated, so it's a bit more complex, but you get the idea.
By default, BASIC will set PAGE to OSHWM (OS High Water Mark - see p431). But you can move PAGE up and down to you heart's content. Remember, BASIC and the MOS are independent of each other!
Now, for added confusion, on a Master that memory used by Paged ROMs is hidden away elsewhere by the MOS, so OSHWM will generally stay around &E00.
And if you're running on a 6502 second/co processor, I believe OSHWM is at &800.
OSHWM is a moveable feast!
Statistics: Posted by james — Sat Feb 17, 2024 10:50 am
