Data Random Access Memory (RAM)

As its name implies, data random access memory (DATA RAM) is used for the temporary storage of data by programs.

The RAM is laid out as shown below:

Data RAM bank organisation

RAM chip organisation

In order to address a 4-bit character of DATA RAM, the programmer first uses a “DCL” instruction to choose one of a maximum of eight DATA RAM BANKS.

An eight bit address is then sent via an “SRC” instruction which chooses one of four DATA RAM CHIPS within the DATA RAM BANK, one of four 16-character DATA RAM REGISTERS within the DATA RAM CHIP, and one of 16 4-bit characters within the DATA RAM REGISTER.

Within any particular DATA RAM BANK, then, addresses 0 - 63 indicate which of the 64 directly addressable characters of DATA RAM CHIP 0 is to be addressed,
addresses 64 - 127 correspond to the characters of CHIP 1,
addresses 128 - 191 correspond to CHIP 2, and
addresses 192 - 255 correspond to CHIP 3.

In addition, each DATA RAM REGISTER has four 4-bit STATUS characters associated with it. These status characters may be read and written like the data characters, but are accessed by special instructions as described here and here