Memory in the Tandy 1000 Series : Why More is Not Always Better

All true Tandy 1000s (true Tandys do not include any RLX or RSX machines) come shipped from the factory with variable amounts of RAM, and all can be upgraded.  Here is the basic chart :

128K - Tandy 1000/A
256K - Tandy 1000HD, 1000EX, 1000SX (also Tandy 1000/A)
384K - Tandy 1000SX, 1000SL (also Tandy 1000/A/HD, 1000EX, 1000HX)
512K - Tandy 1000SL/2, 1000RL/RL-HD (also Tandy 1000/A)
640K - Tandy 1000TX, 1000TL, 1000TL/2, 1000TL/3 (all other upgraded Tandys)
768K - Tandy 1000RL/RL-HD, 1000TX, 1000TL, 1000TL/2, 1000TL/3 (upgraded)

(all systems mentioned in parentheses are systems upgraded with RAM expansion cards, boards or had RAM installed in mainboard sockets after the system came from the factory)

Now you might think that more RAM is always better, but consider a few of these games :

King's Quest, Touchdown Football - The PCjr. Only Booters -versions of these games assume that the video memory is at the top or within the first 128KB of RAM, as it would be on a PCjr.  This assumption is also true for the Tandy 1000/A, but vanishes when you upgrade the RAM with an expansion card. King's Quest will not show any graphics in a Tandy 1000 with more than 128KiB of RAM.

The Tandy 1000 HD comes with one of these cards pre-installed but can be removed to bring the system back down to 128KB. The hard drive controller card that comes with the system also uses DMA, so it may also have to be removed. You can remove the DMA controller chip in the Tandy 1000 SX but this will limit you to 128KiB. 

It is not advisable to run the PCjr. booter versions of King's Quest and Touchdown Football in a 128KiB Tandy 1000. 

Demon Attack - The Tandy version of this game, which unlike the PCjr. version came on disk, will work with RAM up to 640KB, but will fail to work if 768KB is in the system.  This was well before any idea was given to 768KB.  One-on-One and Ultima V will fail to run with 768KB.

Demon's Forge - The Mastertonic 1987 PC port of this game supports 16-color Tandy graphics only if there is no DMA chip in the system.  Otherwise it supports 4-color CGA graphics.  The Tandy 1000/A, EX and HX all do not come with DMA. The official Memory Expansion Plus upgrade board came with a DMA controller but modern upgrade boards do not, so they will not have that limitation. 

Demon Stalkers, Duck Tales : Quest for the Gold, Fire King, Harley-Davidson: The Road to Sturgis, The Three Stooges, The Simpsons : Arcade and Snow Strike - These late 1980s games will show no graphics, garbled graphics or fail to work if the RAM has been upgraded beyond 640KB.  Apparently these games were not programmed with the idea that you could expand a Tandy beyond 640KB, despite the TX already being released by this time.  To play these games, you need to remove the RAM upgrades in a system with 768KB.  Most require 512KB or 640KB, so apparently only two possibilities were envisioned for these games.  Alternatively, you can use their CGA command line arguments (C or CGA usually) to play the games with CGA graphics and Tandy sound.