After the complete instruction is stored in the instruction and/or
data input/output register(s), the instruction decoder transforms
the instruction operation code into the address of the appropriate
instruction-execution routine contained in the internal microprogram.
The microprogram then branches to the specified internal address
to initiate execution of the instruction. The resulting execution
routine comprises one or more microinstructions that implement
the required functions.
For example, the first microcycle of an Extension Register Add
Instruction (ADE) causes the contents of the extension register
to be gated onto the read bus, transferred to the write bus via
the bus control logic, and then written into the data input/output
register. The next microcycle causes the contents of the accumulator
to be gated onto the read bus, the contents of the read bus to
be added to the contents of the data input/ output register via
the ALU, and the resultant output of the ALU to be written into
the accumulator via the write bus. The final step of the execution
routine is a jump back to the fetch routine to access the next
instruction.
The figure below illustrates a typical SC//MP system configuration. The SC//MP is shown interconnected to three memory devices to form a stand-alone 4-device system that provides 256 words of read/write memory (RAM) and 2048 words (ROM) for program storage.
Stand-alone 4-device system
(full size 13KB)
