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

• MSX1s system can see up to 64kB. It is called the main Ram and can be only located in following aeras. Ram on separate area or in other slots will be usable only for specific applications.
  • 8kB MSX1: E000h-FFFFh
  • 16kB MSX1: C000h-FFFFh
  • 32kB MSX1: 8000h-FFFFh
  • 64kB MSX1: 0000h-FFFFh
• All MSX2s have all 64kB Ram, or more by using a memory mapper. You can add an expansion without memory mapper but it will usable only for specific applications. Note that if a memory mapper is added, it will not necessarily be selected at start up. It depends on the location of the internal RAM.
• MSX2+ computers select always the first extra 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 select its internal Ram.
• MSX-DOS2 select the bigger memory mapper as main Ram except on MSX turbo R.

8kB and 16 kB RAM expansion cartridges

These cartridges were designed to upgrade a MSX1 with 8kB or 16kB of memory. They allow to increase the memory allocated to BASIC. 8kB expansion is designed for the Casio PV-7. This computer is an exeption, 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 maintly designed to upgrade a MSX1 to 64kB. They allow to use the MSX-DOS for example.

At first sight an 64kB expansion may seem useless because an 32kB or 48kB expansion is sufficient to upgrade to 64kB but it is nevertheless preferable for MSX that have internal RAM in a slot higher than cartridge slots because in this case the main Ram will be in one piece in the same slot. This improves compatibility with many European software because many games have been developed for this configuration.

64kB expansions cannot be used to upgrade main memory of an 64kB MSX to 128kB because the system does not take into account the Ram that there is more than 64ko. Since MSX2, we have to use a memory mapper to exceed 64kb.

Following diagram shows what RAM the system selects as main memory in initialisation of an MSX1 that have 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 selected by default under BASIC environment. 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 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 fit for all MSX1s.

While this 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 its segments at start and use BMMS before execute your software.

Memory Mapper expansion cartridges

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

Note: ✓ in MSX1 column indicates that the Memory Mapper is able to initialise its pages at start itself.