RAM disk capacity

[equinox]

I was surprised to notice recently that the 128k Speccies don't all have the same size of RAM disk. If you try this on a 128 or +2:

Code: Select all

FOR n=0 TO 99: PRINT n: SAVE ! STR$ n CODE 0,1024: NEXT n

it gets up to number 72 before failing with "4 Out of memory". However, running the equivalent on a +2a or +3:

Code: Select all

FOR n=0 TO 99: PRINT n: SAVE "m:"+STR$ n CODE 0,1024: NEXT n

only reaches 29.

Of course the +2a and +3 have a significantly different ROM from the 128 and +2, but it's the same size ROM. I assumed the RAM banks were basically empty when you switched on (other than the screen display, system variables etc.), so why the big difference in capacity?

[zup]

+3 uses 2k of bank 1 as a disk cache, so they are unavailable as RAM disk.

[AndyC]

It's not actually the same size ROM, the +2A/+3 has 64K of ROM, where the +2 has just 32K, although I don't think that matters here.

The RAMdisk on the +2 is a very minimal implementation and can use all 5 extra 16K pages on the 128. I'd assume the directory structure is also pretty minimal. The RAMdisk on the +3 is a more fully featured disk emulation as it's provided by +3DOS, but that's limited to only 4 of the banks as +3DOS reserves page 7 for internal use and the second screen, it also reserves some of the RAM for the disk cache (configurable by calling DOS SET 1346)

[equinox]

Hmm, so, relatedly: are there any games that work on a 128/+2 but not on a +2/+3 purely because there isn't enough RAM?

[AndyC]

Hmm, so, relatedly: are there any games that work on a 128/+2 but not on a +2/+3 purely because there isn't enough RAM?

I doubt it. The Syntax for using the RAMdisk is entirely incompatible between the two, so that'd break thing in BASIC long before running out of RAM.

In a pure m/code game, you can happily trash all the +3DOS reserved memory, unless you're loading from disk. But if someone's done a disk conversion, they've likely already worked around any +3DOS issues.

[spider]

Ah yes "!" vs "M" if I recall.

As one bank is used by +3DOS , I wonder what happens with that on the +2A/B , if its got anything in it or used, as in for the ram-disk.

[TMD2003]

Hmm, so, relatedly: are there any games that work on a 128/+2 but not on a +2/+3 purely because there isn't enough RAM?

I spy a challenge on the horizon...

[AndyC]

Ah yes "!" vs "M" if I recall.

As one bank is used by +3DOS , I wonder what happens with that on the +2A/B , if its got anything in it or used, as in for the ram-disk.

It's the same. +3DOS still reserves some for a disk cache and keeps the second screen free regardless of whether a disk drive is attached or not.

[1024MAK]

The +2A/+2B/+3/+3B use essentially the same ROM code, hence work the same (more or less) when it comes to common features.

Mark

[equinox]

I spy a challenge on the horizon...

Yeah, unfortunately my Rockfall 3 mod may prove to be the first one. So I'm going to have to do a standard +3 multiload version too (in which case I might as well also allow it to work on a 48K or +2a with tape or +D)... zzz...
At least if I can work out that bloody issue in the other thread, else it's all off.

I can remember a very few utilities that worked with the RAM disk, btw. The 128 version of the (Wham!) Music Box was one (and actually used BASIC for the RAM load & save), Superfile was another. Art Studio 128 offers RAM save but there's presumably no BASIC involved there.