Education for ALL: SAP-1 Architecture

The Simple-As-Possible (SAP)-1 computer is a very basic model of a microprocessor explained by Albert Paul Malvino. The SAP-1 design contains the basic necessities for a functional Microprocessor. Its primary purpose is to develop a basic understanding of how a microprocessor works, interacts with memory and other parts of the system like input and output. The instruction set is very limited and is simple.

SAP (Simple-As-Possible)-1 is the first stage in the evolution toward modern computers.

Figure: Architecture of SAP-1 Microprocessor/Computer

SAP is Simple-As-Possible Computer. The type of computer is specially designed for the academic purpose and nothing has to do with the commercial use. The architecture is 8 bits and comprises of 16 X 8 memory i.e. 16 memory location with 8 bits in each location, therefore, need 4 address lines which either comes from the PC (Program Counter which may be called instruction pointer) during computer run phase or may come from the 4 address switches during the program phase. All instructions (5 only) get stored in this memory. It means SAP cannot store program having more than 16 instructions.

SAP can only perform addition and subtraction and no logical operation. These arithmetic operations are performed by an adder/subtractor unit.

There is one general purpose register (B register) used to hold one operand of the arithmetic operation while another is kept by the accumulator register of the SAP-1.

In addition, there are 8 LEDs which work as output unit and connected with the 8 bit output register.

All timely moment of data or activities are performed by the controller/sequencer part of the SAP-1.

Program Counter

It counts from 0000 to 1111.
It signals the memory address of next instruction to be fetched and executed.

Inputs and MAR (Memory Address Register)

During a computer run, the address in PC is latched into Memory Address Register (MAR).

The RAM

The program code to be executed and data for SAP-1 computer is stored here.
During a computer run, the RAM receives 4-bit addresses from MAR and a read operation is performed. Hence, the instruction or data word stored in RAM is placed on the W bus for use by some other part of the computer.
It is asynchronous RAM, which means that the output data is available as soon as valid address and control signal are applied.

Instruction Register

IR contains the instruction (composed of OPCODE+ADDRESS) to be executed by SAP1 computer.

Controller-Sequencer

It generates the control signals for each block so that actions occur in desired sequence. CLK signal is used to synchronize the overall operation of the SAP1 computer.
A 12-bit word comes out of the Controller-Sequencer block. This control word determines how the registers will react to the next positive CLK edge.

Accumulator

It is a 8-bit buffer register that stores intermediate results during a computer run.
It is always one of the operands of ADD, SUB and OUT instructions.

Adder/Subtractor

It is a 2's complement adder-subtractor.
This module is asynchronous (unclocked), which means that its contents can change as soon as the input words change.

B-register