Chapter 13 ½ Bit-Sliced Microprocessor of the Am2900 Family : The Am2901/2909 169

Am2901: Four-Bit Bipolar Microprocessor Slice

The device, as shown in Fig. 2, consists of a 16-word by 4-bit two-port RAM, a high-speed ALU, and the associated shifting, decoding, and multiplexing circuitry. The 9-bit microinstruction word is organized into three groups of 3 bits each and selects the ALU source operands, the ALU function, and the ALU destination register. The microprocessor is cascadable with full lookahead or with ripple carry, has three-state outputs, and provides various status flag outputs from the ALU. Advanced low-power Schottky processing is used to fabricate this 40-lead LSI chip.

Architecture

A detailed block diagram of the bipolar microprogrammable microprocessor structure is shown in Fig. 3. The circuit is a 4-bit slice cascadable to any number of bits, Therefore, all data paths within the circuit are 4 bits wide. The two key elements in the Fig. 3 block diagram are the 16-word by 4-bit two-port RAM and the high-speed ALU.

Data in any of the 16 words of the random-access memory (RAM) can be read from the A port of the RAM as controlled by the 4-bit A address field input. Likewise, data in any of the 16 words of the RAM as defined by the B address field input can be