Pamantasan ng Lungsod ng Maynila(University of the City of Manila)

Intramuros, ManilaCollege of Engineering and Technology

Computer Studies Department

Computer Organization, Architecture and Machine Level Programming

SAP-1 MODIFICATION

Submitted by:

Aromin, Denisse Aubrey L.Buraga, Vincent Paul B.Dianzen, Fruzzel Paul P.

Francia, Munich J.Garillo, Lois Eunice R.

Jimenez, Mary Grace S.Tolentino, Joshua Nico V.

Submitted to:Prof. Melannie Mendoza

March 11, 2011

INTRODUCTIONThe Simple-As-Possible (SAP)-1 computer is a very basic model of a microprocessor explained by Malvino. The SAP-1 design contains the basic necessities of 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 SAP-1 is very limited and simple. Its main purpose is to introduce all the crucial ideas behind computer operation without burying in unnecessary details. But even simple computer like SAP covers many advanced concepts. SAP-1 is the evolution toward modern computers. Be ready to SAP-1 and you will be ready to SAP-2.

The features of SAP-1 computer are:

○ RAM ○ Instruction Registers ○ Program counter ○ Memory Address Register (MAR)○ Arithmetic Logic Unit○ Controller-Sequencer

○ Accumulator

○ B Register

○ Output Register

○ Binary Display

with:

○ AND Accumulator

○ OR Accumulator

○ Exclusive OR Accumulator

○ Rotate Accumulator Left

○ Rotate Accumulator Right

The instruction format of SAP-1 Computer is:

(XXXX) (XXXX)

The first four bits make the opcode while the last four bits make the operand (address).

INSTRUCTION SET CONSISTS OF FOLLOWING INSTRUCTIONS:

Mnemonic OPERATION OPCODE

LDA Load addressed memory contents into accumulator.

0000

ADD Add addressed memory contents to accumulator. 0001

SUB Subtract addressed memory contents from accumulator.

0010

OUT Load accumulator data into output register. 1110

HLT Stop processing. 1111

ANA AND Accumulator contents with designated register.

1010

ORA OR Accumulator contents with designated register.

0101

XRA Exclusive OR accumulator. 0100

RAL Rotate Accumulator Left. 1000

RAR Rotate Accumulator Right. 1001

MACHINE CYCLE AND INSTRUCTION CYCLESAP1 has six T-states (three fetch and three execute cycles) that are reserved for each instruction. Not all instructions require all the six T-states for execution. The unused T- state is marked as NoOperation (NOP) cycle.

Each T-state is called a machine cycle for SAP1. A ring counter is used to generate a T-state atevery falling edge of clock pulse. The ring counter output is reset after the 6th T-state.

FETCH CYCLE – T1, T2, T3 machine cycle

EXECUTE CYCLE – T4, T5, T6 machine cycle

ARCHITECTURE1. Program Counter (PC)

• Implemented in “pc.v” file.

• It is part of the control unit and counts from 0000 to 1111 and it signals the memory address of next instruction to be fetched and executed. It is reset to 0000 before each computer run. When the computer runs, the program counter sends address to 0000 to the memory and it is incremented to get 0001.

2. Input and MAR (MAR)

• Implemented in “inputMAR.v” file

• Below the program counter is the input and MAR block. It includes the address and data switch registers. The MAR is part of the SAP-1 memory. During a computer run, the address in PC is latched into Memory Address Register (MAR).

3. RAM

• Implemented in “mem16k.v” file.

• The RAM is a 16 x 8 static TTL. The program code to be executed and data for SAP1 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.

4. Instruction Register (IR)

• Implemented in “ir.v” file

• It is part of the control unit. To fetch an instruction from the memory the computer does a memory read operation. This places the memory

address contents in the W bus. IR contains the instruction (composed of OPCODE+ADDRESS) to be executed by SAP1 computer.

5. Controller- Sequencer

• Implemented in “cu.v” file.

•This resets the program counter to 0000 and wipes out the last instruction in the instruction register. 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. The 12 wires carrying the control word are called the control bus.

6. Accumulator

• Implemented in “accumulator.v” file.

• It is a 8 bit buffer register that stores intermediate results during a computer run and has two outputs. The two-state output goes to the W bus. Therefore the 8 –bit accumulator word continuously drives the adder- subtracter. The same word appears on the W bus when Ea is high.

• It is always one of the operands of ADD, SUB and OUT instructions.

7. Adder-Subtracter

• Implemented in “addersubtracter.v” file.

• 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.

• When Su is low, the sum out of the adder-subtracter is S = A + B.• When Su is high the difference appears: A = A+B’

8. B Register

• Implemented in “register.v” file.

• It is 8 bit buffer register which is primarily used to hold the other operand (one operand

is always accumulator) of mathematical operations.

• The two-state output of the B register drives the adder-subtracter, supplying the number to be added or subtracted from the contents of the accumulator.

9. Output Register

• This registers hold the output of OUT instruction.

• Often called an output port because processed data can leave the computer through this register.

10. Binary Display

• It is a row of eight Light Emitting Diodes to show the contents of output register.

• The LED connects to one flip-flop of the output port, the binary display shows us the contents of the output port.

INSTRUCTION SET

Programming a computer means loading the instructions step-by-step into the memory before the start of the computer run. Before you can program a computer , you must learn its instruction set.

LDA

It stands for “load the accumulator.” A complete LDA instruction includes the hexadecimal address of the data to be loaded.

ADD

ADD is another SAP-1 instruction. A complete ADD instruction includes the address of the word to be added. The addressed RAM word goes to the B Register and the adder-subtracter output to the accumulator.

SUB

A complete SUB instruction includes the address of the word to be subtracted. The addressed RAM word goes to the B Register and the adder-subtracter output to the accumulator.

OUT

The instruction OUT tells the SAP-1 computer to transfer the accumulator contents to the output port. After OUT has been executed, we can see the answer to the problem being solved.

HLT

HLT stands for HALT. This instruction tells the computer to stop processing data. HLT marks the end of a program, similar to the way a period marks the end of a sentence. HLT is complete by itself.

ANA

ANA means to AND the accumulator contents with the designated register. The result is stored in the accumulator. Two ANA instructions are available in SAP-2: ANA B ad ANA C.

ORA

ORA is the mnemonic for OR the accumulator with the designated register. The two ORA instructions in SAP-2 are ORA B and ORA C.

XRA

XRA means xor the accumulator with the designated register. The SAP-2 instruction set contains XRA B and XRA C.

RAL

RAL is the mnemonic for rotate the accumulator left. This instruction will shift all bits to the left and move the MSB into the LSB position.

RAR

RAR stands for rotate the accumulator right. The bits shift to the right, the LSB going to the MSB position.

FETCH AND EXECUTE CYCLE

SAP 1 ARCHITECTURE

PROGRAM COUNTER

Input AND MAR

16x8RAM

INSTRUCTION REGISTER

CONTROLLER SEQUENCER

4 4

412

8

4

4

4

8

8

W BUS

ACCUMULATOR

ADDER/SUBTRACTOR/ANA/ORA/RAR/RAL/XRA/

XNA

8

8

8

8

8

B REGISTER8

OUTPUT REGISTER

BINARY DISPLAY

88

CLK

CLK

CLR

CLR

CP

CLKCLREP

LM

CLK

CE

LI

CLKCLREI

LA

CLK

EA

SU

EU

LB

CLK

CLK

LO

CP EP LM CE LI EI LA EA LB LO SU EU

CONCLUSION

We became familiar with the working of a 8-bit microprocessor and the way each module

contributes to the overall functioning of the computer. Although the concept of SAP1 is very

simple, the knowledge gained while designing it can be extended to design of more complex

microprocessors.

REFERENCE

Albert Paul Malvino. Digital Computer Electronics – An Introduction to Microcomputers:

SAP-1(Page 140). Tata McGraw-Hill Publishing Company Limited 1990.