Important points to take into account for buying a RAM expansion

Before buying a memory expansion, you need to pay attention to several points.

• MSX1 system can by default only see up to 64kB. It is called the main RAM and can only be located in the following areas. RAM in different areas or in other slots will only be usable for specific applications.
  • 8kB MSX1: E000h-FFFFh
  • 16kB MSX1: C000h-FFFFh
  • 32kB MSX1: 8000h-FFFFh
  • 64kB MSX1: 0000h-FFFFh
• a memory mapper expansion cartridge can be added to a MSX1, but the MSX itself will not be able to initialise it. So either the memory mapper must be able to self-initialise or you must manually initialise it each time.
• All MSX2s have at least 64kB RAM. If they have exactly 64kB RAM they may or may not have a memory mapper, if they have more then 64kB they will have a memory mapper. You can add an expansion without memory mapper but it will only be usable for specific applications. Note that if a memory mapper is added, it will not necessarily be selected at start up. It depends on the slot location of the internal RAM, compared to the slot the expansion is installed in.
• MSX2+ computers always select the first additional memory mapper at start because the internal RAM is in a secondary slot 3-x. (if you do not place another RAM expansion in a lower slot.)
• MSX Turbo R always selects its internal RAM.
• MSX-DOS2 selects the largest memory mapper as main RAM except on a MSX Turbo R.

8kB and 16 kB RAM expansion cartridges

These cartridges were designed to upgrade a MSX1 with 8kB to 16kB or a MSX1 with 16kB to 32kB of memory. On those systems, they will also increase the amount of free memory in BASIC. The 8kB expansion is designed for the Casio PV-7. This computer is an exception, avoid expansions from 16kB to 48kB for it. These are only usable for very specific cases on this computer.

32kB to 64 kB RAM expansion cartridges

These cartridges were mainly designed to upgrade an MSX1 to 64kB. This allows the use of MSX-DOS for example. The 32kB expansions will upgrade a 32kB MSX1 to 64kB. The 48kB expansion will upgrade a MSX1 with 16 or 32kB to 64kB and the 64kB expansion will upgrade any MSX1 with less then 64kB to 64kB.

At first sight an 64kB expansion may seem overkill as either a 32kB or 48kB expansion would be sufficient to upgrade to 64kB. It is however preferable to have all 64kB in the same slot as some software titles blindly assume this to be the case. Having the memory split between different slots can cause incompatibility problems with a few (often European) titles.

The 64kB expansions cannot be used to upgrade main memory of a 64kB MSX to 128kB, as that requires a memory mapper which these cartridges lack.

The following diagram shows what RAM the system selects as main memory on an MSX1 that has its RAM in slot 0. It is an example for illustrative purposes as that is the case for most MSX1 systems with less than 64kB of RAM. However, the memory can in reality be located in any slot.

Green areas are RAM regions selected by default when running BASIC. Green and light green areas are RAM selected by default under MSX-DOS1 (system with disk). Yellow areas are ignored by the system. These areas will only be usable for specific applications.

As you can see in the diagram, a 16kB expansions are designed for 16kB MSXs, 32kB expansions are designed for MSXs with 32kB, 48kB expansions are designed for MSXs with 16kB or 32kB. As for 64kB RAM expansions, they are suitable for all MSX1s.

While having memory split between slots is supported in the MSX specification (and some MSX systems even have the internal memory split over two slots), when the main memory is located in different slots it is known to cause problems with software that blindly assumes that all the RAM will be in a single slot. To avoid this problem choose an expansion with memory mapper that initialise itself at start and use BMMS before executing your software.

External memory mapper cartridges in machines with T9769 MSX-Engine chip

This chip is usually installed in the Turbo R machines, and, when reading the mapper registers at ports &HFC to &HFF, pulls unused mapper bits to +5V with enough current causing considerable heat and current consumption. Here's the excerpt from the article in the MSX Computer and Club Magazine #65 (page 37, translated from Dutch):

Due to an error in the engine Toshiba T9769, the non-connected data bits are read back as a 1 (+5 Volt) ... In itself this is not bad, the engine S1985 (Sony F700) does this, but with the T9769 the unused data bits are connected low-ohm to the +5 Volt. When a second mapper is used in the system, that is larger than the internal mapper, this second mapper will attempt to pull these data bits down. If the buffer placed in the external mapper is strong enough, this configuration will indeed work. Apparently everything works normally, but at that moment there is a large current, which generates a lot of heat in the T9769 engine.

Memory Mapper expansion cartridges

These expansions are primarily meant for MSX2 and MSX2+ systems. The MSX1 BIOS is not able to initialise them, but you can manually initialise them or some cartridges can self-initialise.

For more information about how a Memory Mapper works, see Memory Mapper.

Note: ✓ in MSX1 column indicates that the Memory Mapper is able to initialise its pages to 3, 2, 1, 0 at the system start (hard reset or power cycle) itself.