Microcode - Examples of Microprogrammed Systems

Examples of Microprogrammed Systems

• In common with many other complex mechanical devices, Charles Babbage's analytical engine used banks of cams to control each operation, i.e. it had a read-only control store. As such it deserves to be recognised as the first microprogrammed computer to be designed, even if it has not yet been realised in hardware.
• The EMIDEC 1100 reputedly used a hard-wired control store consisting of wires threaded through ferrite cores, known as 'the laces'.
• Most models of the IBM System/360 series were microprogrammed:

• The Model 25 was unique among System/360 models in using the top 16k bytes of core storage to hold the control storage for the microprogram. The 2025 used a 16-bit microarchitecture with seven control words (or microinstructions). At power up, or full system reset, the microcode was loaded from the card reader. The IBM 1410 emulation for this model was loaded this way.
• The Model 30, the slowest model in the line, used an 8-bit microarchitecture with only a few hardware registers; everything that the programmer saw was emulated by the microprogram. The microcode for this model was also held on special punched cards, which were stored inside the machine in a dedicated reader per card, called "CROS" units (Capacitor Read-Only Storage). A second CROS reader was installed for machines ordered with 1620 emulation.
• The Model 40 used 56-bit control words. The 2040 box implements both the System/360 main processor and the multiplex channel (the I/O processor). This model used "TROS" dedicated readers similar to "CROS" units, but with an inductive pickup (Transformer Read-only Store).
• The Model 50 had two internal datapaths which operated in parallel: a 32-bit datapath used for arithmetic operations, and an 8-bit data path used in some logical operations. The control store used 90-bit microinstructions.
• The Model 85 had separate instruction fetch (I-unit) and execution (E-unit) to provide high performance. The I-unit is hardware controlled. The E-unit is microprogrammed; the control words are 108 bits wide on a basic 360/85 and wider if an emulator feature is installed.

• The NCR 315 was microprogrammed with hand wired ferrite cores (a ROM) pulsed by a sequencer with conditional execution. Wires routed through the cores were enabled for various data and logic elements in the processor.
• The Digital Equipment Corporation PDP-11 processors, with the exception of the PDP-11/20, were microprogrammed.
• Many systems from the Burroughs were microprogrammed:

• The B700 "microprocessor" executed application-level opcodes using sequences of 16-bit microinstructions stored in main memory, each of these was either a register-load operation or mapped to a single 56-bit "nanocode" instruction stored in read-only memory. This allowed comparatively simple hardware to act either as a mainframe peripheral controller or to be packaged as a standalone computer.
• The B1700 was implemented with radically different hardware including bit-addressable main memory but had a similar multi-layer organisation. The operating system would preload the interpreter for whatever language was required. These interpreters presented different virtual machines for COBOL, Fortran, etc.

• Microdata produced computers in which the microcode was accessible to the user; this allowed the creation of custom assembler level instructions. Microdata's Reality operating system design made extensive use of this capability.
• The Nintendo 64's Reality Co-Processor, which serves as the console's graphics processing unit and audio processor, utilized microcode; it is possible to implement new effects or tweak the processor to achieve the desired output. Some well-known examples of custom microcode include Factor 5's N64 port of the Indiana Jones and the Infernal Machine, Star Wars: Rogue Squadron and Star Wars: Battle for Naboo.
• The VU0 and VU1 vector units in the Sony PlayStation 2 are microprogrammable; in fact, VU1 was only accessible via microcode for the first several generations of the SDK.