root42 wrote on 2020-07-17, 19:11:

My 286 BIOS has a Scratch RAM option. What's that? Never encountered that before!

Bildschirmfoto 2020-07-17 um 21.10.06.png

That option (maybe with slightly different naming) exists up to 486 boards until mandatory shadow was introduced. There is a 257 byte BIOS data area between 40:0 and 50:1 (50:0 is the "print screen is active" flag). This area got too small for some advanced BIOS features, so the AMI BIOS needs more memory. A typical use case for extra RAM is to store the parameters of user-specified Type 47 hard disks, but I'm not sure your BIOS already supports it. The usage for hard drive parameter is the reason this setting is sometimes named "Drive type 47 data area".

The two choices with this option are to either use the memory space that is meant for interrupt vectors C0 to FF (30:0..40:0), which is "free" unless some program tries to use the same memory area, so it has limited compatibility. The other choice reports only 639 KB instead of 640KB RAM to DOS (the BIOS can only report full kilobytes), so the last kilobyte (9FC0:0..A000:0) is free for use by the BIOS. The impact is a whole kilobyte less DOS memory, but you should not get compatibility issues, as the memory is properly allocated.

This option disasspeared from setups with the introduction of compressed BIOSes that get decompressed during power-on and have the decompressed BIOS stored in shadow RAM. As shadow RAM can be patched on-the-fly during the POST, the BIOS can store hard drive parameter tables in its own segment instead of using some area in conventional memory. Most BIOSes with optional shadow already used the shadow RAM instead of "30:0" or "DOS 1K" as long as shadowing is enabled.

That version of amibios does in fact support type 47. I've never had an issue using the 30 seg

According to Ralf Brown's Interrupt List, AMI BIOS only uses vectors C0..C7, and the only conflicting software is:

IBM ROM BASIC - used while in interpreter
APL*PLUS/PC

There might be issues when manually including memory addresses for emm386 to use. I experienced that one and Switched back to dos 1kb

Marco wrote on 2020-07-18, 19:27:

There might be issues when manually including memory addresses for emm386 to use. I experienced that one and Switched back to dos 1kb

Which is kind of a non issue on my 286. 😀 But I get your point...