After starting designing micro-instruction sequences for memory access operations, I encountered some difficulties with the scheme presented in previous log entry. Thus I had to modify this scheme; the modifications were not very big though.

Following changes were made:

1) added 16-bit Memory Data Buffer register -- its primary use is to temporarily store direct offsets;

2) final address source selector was widened from 1-of-2 to 1-of-4, so that all needed address sources are available;

3) added selector of base address source - now base address can be in Memory Pointer, or in Memory Address Buffer -- this is most useful in multi-word data transfers between registers and memory.

Below is the scheme itself, and after it there are some notes about it:

(By the way, the Excel turned out to be quite useful as line drawing tool for creation of the above scheme)

Register layout and  function:

There are 16 addressable registers in 2 blocks, 8 General Purpose Registers (GPR) and Memory Pointers (MP). Each group has dedicated arithmetic unit, for GPR this is 16-bit Main ALU, for the Memory Pointers it is a 32-bit adder.

There is also the 16-bit Instruction Register, which holds currently executed instruction, 16-bit Status Register, which is very special in that its bits can be set and cleared independently, and which is manipulated in its own special way, 16-bit Memory Data Buffer, which is used in some kinds of memory access operations, and 32-bit Memory Address Buffer, which is the source of address in most of the memory access operations.

Busses:

There are 3 main busses -- 

1 - the 16-bit Main Data Bus, it is used for general data exchange,

2 - 32-bit Address Bus, providing address for accessing memory location,

3 - 16-bit Instruction Bus, that is kind of offshoot of the main bus, but can be cut off, so some register transfers can be combined with the fetching of new instruction.