Lowest usable address on Page 3

16 bit value stored at addresses 64586 and 64587 points to the beginning of reserved area of Dos and Basic. It should be more or less the last 10 kb.

I’d say the options are:
- Some areas you could use like the starting at “Data area used by CIRCLE statement” + the Paint space, or the starting at VOICAQ if you don’t use the PLAY, where you have 384 bytes.
https://www.konamiman.com/msx/msx2th/th-ap.txt
- At the start of your program, read the SP value, and add enough (substracting to SP) so the new SP value is just below your first desired fixed usable address. Then restore at program exit.

The cartridges uses their own ROM so they don’t use the RAM segments, but have in mind that they could be mapped into some page at some point, i.e. in DOS at VBLANK the BIOS is set at page 0, but also the DiskROM is mapped at page 1, so if you want to access RAM while VBLANK in DOS, and use the page 1, you would be accessing really to the DiskROM. It depends on the system.

• In Basic the SP is set at about #DB80. That leaves about 7000 bytes in segment 0.
• In DOS1.x the SP is set at #D5D1. That leaves about 5585 bytes in segment 0.

I wanted to have a large block of free data (8k+) in Page 3 so I think I'd be better off switching Page 3 to another segment and using the same locations for the all the BIOS variables.
OR (even better)... temporarily storing the Data between #D500 and #EFFF in another segment so I can overwrite this area in Page 3 segment 0. And restore the data just before exiting the application.
I think BDOS commands will not use the #D500 and #EFFF area.

Thanx guys! You've been a lot of help. I think I've got what I need to proceed, but any other suggestions are welcome.

Incidently, I was asking myself a similar question this weekend. And I mapped out this area using the MSX system sourcefiles by zeilemaker54. This map is for Disk Basic 1.0 only.

There's more than just the system variables in this area. Without diskdrive, it's the same for every MSX machine.

HIMEM   F380  start of system area
NULBUF  F16A
FILTAB  F168
MEMSIZ  F177
STKSTP  F0A0
SAVSTK  F09E
VARTAB  8003

If you substract VARTAB from STKSTP, and then substract 14 you will get the amount of bytes free in Basic. So that's F0A0h - 8003h - 14 = 28815. That value of 14 is hard coded in the Basic rom.

When a diskdrive is present, things get a bit more complicated, and manufacturer specific. Values below are for Philips MSX2. I'll explain other manufacturers later.

var     addr    len     description
        F380            system area
        F1C9    439     disk system area
        F1C1    8       disk driver area (manufacturer specific)
        F197    42      number of drives * drive parameter block size (21 bytes)
        EF97    512     sectorbuffer (512 bytes)
        ED97    512     datasector buffer (512 bytes)
        EB97    512     dirsector buffer (512 bytes)
        E596    1537    FAT buffer drive 0 (3 * 512 bytes + 1)
        DF95    1537    FAT buffer drive 1 (3 * 512 bytes + 1)
        DE92    259     7 fcb's * 37 bytes
HIMEM   DE79    25      bload/bsave work area
NULBUF  DC70
FILTAB  DC63
MEMSIZ  DC61
STKSTP  DB99
SAVSTK  DB97
VARTAB  8003

To check, DB99h - 8003h - 14 = 23432 bytes free. If you boot with ctrl you get only one drive. It will save a drive parameter block (21 bytes) and a FAT buffer (1537 bytes). You'll gain 1558 bytes, so that will be 24990 bytes free.

The differences will be in the manufacturer specific driver area. This driver resides in the diskrom and different drivers require a different amount of memory. Philips uses 8 bytes, Sony 9 and Panasonic or Sanyo 26. That will result in 23432, 23431 or 23414 bytes free.

There are a few diskroms that only support single sided drives, like the Philips VY-0010 or the Spectravideo SVI-738 X'PRESS. These will need only 1025 byte FAT buffers. And you will end up with 24456 bytes free.

When booting into DOS the map will be different as well, because MSXDOS.SYS will be loaded into high memory.