Right now I have 128K and the video takes the top 16K leaving 112K. I read in another thread that Tandy video uses the end of memory so if you expand it, it will use it from the end. If I expand it to 640K, then it would use the 16K at 624K-640K.
So a minor correction/clarification: The Tandy 1000 can only use the RAM on the motherboard for video memory. (It's basically "owned" by the video system and shares it with the CPU, not vice-versa.) How memory expansion works is there are registers in the video system that lets the 1000 *move* the built-in RAM; at power-on it maps a minimal amount into the Bxxxx page and starts fishing to see if there's expansion RAM installed starting at 0000h. It looks for RAM in 128k pages, up to four of them, and then drops the onboard memory at the end.
So for a lo-tech RAM board you will want switches 1.1-1.8 all on to map RAM from 0-512k.
Q#1 - How does it deal with needing 32K video memory for the modes that need it?
When using a mode with 32k of VRAM it updates the BDOS data area in low RAM to change the soft top-of-memory mark.
Q#2 - Is DOS smart enough to recognize its need for this video at the end, or does the BIOS handle that?
DOS itself, at least contemporary pre-DOS-5 DOS, is fine, (DOS resides at the bottom of available memory, as do most drivers, and programs are loaded from the bottom starting at the first free location) but if you're running software that does stuff with the end of memory then there are incompatibilities you can trip over when the top of memory gets moved around or the stuff left there gets clobbered. A couple examples:
1: If you use DOS 5+ and something like USE!UMBS+DOSMAX to enable loading drivers high using a Tandy video mode will "break" the connection to the upper memory blocks. (This happens because DOS puts this "linked list" kind of structure at the end of conventional RAM that points to the upper memory area; it's weird. And that gets clobbered when you use Tandy graphics because DOS isn't smart enough to move it when the BIOS updates the top of conventional memory pointer) It doesn't (usually) cause a crash, the drivers loaded up there keep working, but DOS can't track that memory anymore or load anything else into it until you reboot.
2: Some disk controllers or other add-on BIOSes steal 1k of memory from the top to use for their purposes. This includes the XTIDE BIOS if you set it to use "Full Operating Mode". This breaks on a Tandy 1000.
Q#3 - Can the banks at 0xA000, 0xC000, 0xD000, or 0xE000 be enabled on the lo-tech? What happens if they are? Will it then use memory above 0xA000 for the video ram?
Based on my experiments with the Tandy 1000EX and HX, which are basically the same, the answer re: A000h is you can go ahead and put something there (It's an option to use it as the EMS window on my combo card for those machines) but you can't do the "704k" trick you can with a CGA PC to get it to show up as main memory because the memory for video will *always* be stolen from the 128k on the motherboard and in a plain 1000/A/EX/HX you are stuck using that RAM as part of main memory. That means you'll have at minimum, always, a 16k "hole" between the top of to contiguous conventional DOS memory and that page you put in the A region. So all it will be good for is loading drivers/whatever high with the apropros software.
0C-E000h are wide open to be used as upper memory blocks, so if you want to use DOS5 or higher go nuts. Because the Lo-Tech card is limited to 64k page granularity you'll probably need to leave one of those pages open for the option ROM if you intend to use a hard drive controller.
Other note is, obviously, if you use the Lo-Tech card you'll be missing a DMA controller. My opinion re: not having one is "could hardly care less", but it does mean you can't use some cards and a few programs that hit the floppy drive on a bare-metal level might not work. (Imagedisk)
