Bit-Slice Design: Controllers and ALUs

Refining the CCU

Last Edit November 2, 1996; May 1, 1999

Interrupt Servicing - Continued

Vector Mapping PROM

Rather than using the main microprogram memory to store a branch table, which requires a full microword to store what is essentially a branch address, it is better to use a vector mapping PROM. A vector mapping PROM is similar to the mapping PROM that was used earlier for the start address of the normal microroutines, with the vector mapping PROM providing the start addresses of interrupt service microroutines.

The index bits from the priority encoder are the address bits supplied to the vector mapping PROM, and the PROM output is a full n-bit address. By using a PROM with tri-state output as the vector map, the map can share the microprogram address input with the mapping PROM and the branch address. The next-address control block must supply an OE'_vector control line in this case. (See Figure 4-6.)

Figure 4-6 Using a vector map

Next Address Control

The next address control can be handled by an Am29811A and an Am25 LS139 decoder. The Am29811 supplies the output enables for the map and pipeline register. By decoding these two signals, the three output enables (map, pipeline, vector) are generated. The Am29811 has an instruction called CJV (Conditional Jump Vector), which operates as any other conditional instruction except that if COND = TRUE, the vector map is enabled and OE'_map amd OE'_pipeline are used to disable the mapping PROM and the pipeline register.

If the microprogram sequencer which was developed in Chapter 3 is modified by making the address register and the counter into one unit with the associated next-address control changes, then the register-counter, the next-address logic, the next-address MUX, the stack, the TOS pointer, the µPC register, and the incrementer exist as a single IC device. This is the Am2910 microprogram sequencer (dubbed "supersequencer"), which can address up to 4K of PROM memory. It is approximately equivalent to three Am2911s, and Am29811, and an Am25LS139. It is referred to as the "supersequencer" because it is powerful enough for most controller applications (see Figure 4-7).

Figure 4-7 AMD's Am2910 Supersequencer - now obsolete - solved many of the CCU construction problems

The Am2910 was so populart that is still exists as a model of a sequencer. Universities build silicon for it, Libraries model it (in Verilog and VHDL), it has been emulated, it has ben built into other larger ICs, in short, it has never left us although AMD no longer supplies a data sheet nor supports it in any way. You cannot buy an Am2910 part. This author still gets requests.