Page 116 - Anatomy of a Robot

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COMPUTER HARDWARE 101
CISC
Complex Instruction Set Computers (CISC) have been the norm since the commercial
introduction of computers. The instructions are much more complex than those in RISC
machines (hence the name). Although a CISC machine may still have ADD and SUB
(subtract) instructions, it may also have MPY (multiply), DVD (divide), ECC (error
checking and correction), and MAC (multiply and accumulate) instructions that per-
form complex calculations. A MPY instruction typically requires a series of ADDs and
SHIFTs. A DVD instruction requires a series of SUBs and SHIFTS. A MAC requires
at least an MPY and an ADD.
It can be very expensive to build the control circuitry within a computer that can man-
age the cycles in such a complex instruction. What most processor designers did was
build a uCode (microcode) engine into the computer processor. Effectively, a small,
very high-speed uCoded RISC processor would be inside the CISC processor. The
uCode program would reside in high-speed Read-Only Memory (ROM) and would exe-
cute a very short series of uCoded machine cycles to carry out the intended CISC
instruction. The uCode for an MPY x,y instruction would look very much like the
following:

uLOAD z, wordlength ; MPY instruction execution.
Loopr: uSHIFTRight x ; Shift x
uTSTskp Carry x ; Was a 1 there?
uADD x,y ; Yes, add Y to answer
uDECJmp z, Loopr ; No, loop til done
uRTN ... ; Finished

The uCode program would execute a series of shifts and adds to accomplish the MPY.
The advantage of uCoding the instruction set is that the CISC hardware could be sim-
plified. Many CISC instructions could be coded with just a few entries in the uCode
ROM. A CISC instruction set might take longer to execute a program, but the compiled
C programs (supplied by the users) would have a smaller number of bytes.
Some uCoded processors enable end users to supply uCode, which can be executed
out of fast RAM inside the uCode engine. This feature is an attempt by the CISC design-
ers to capture some of the advantages of the RISC architecture. End users can effec-
tively make their own instructions up. This is of use if the robot has one or two simple
algorithms that must run faster. It can be very difficult, however, to write uCode. The
documentation is often not very good, and support is often worse.