Tuesday, July 27, 2010


My blog is all about the relations or connections of early microcomputer design to the Simple As Possible-1 (SAP-1) Architecture. In this blog, I talk over or write about the similarities involving the two. I also post images, videos, and links that will also help for better understanding as well as to gather more ideas and information. Hope you enjoy viewing my blog!

To have an overview of the topic, I gather series of information to different references with regards to the main topic.

•What is Computer Architecture?

-The manner in which the components of a computer or computer system are organized and integrated.

*Miriam Webster Dictionary

-The term architecture is used here to describe the attributes of a system as seen by the programmer, i.e., the conceptual structure and functional behavior as distinct from the organization of the data flow and controls, the logic design, and the implementation.

*Gene Amdahl, IBM Journal of R & D, April 1964

-In this book the word architecture is intended to cover all three aspects of computer design-instruction set architecture, organization and hardware.

*Hennessy and Patterson, Computer Architecture: A Quantitative Approach.

A simple thought about SAP-1

The Simple-As-Possible (SAP)-1 computer is a basic model of a microprocessor. 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.

The attributes in SAP-1 computer are

● W bus - A single 8 bit bus for address and data transfer.
● 16 Bytes memory (RAM)
● Registers are accumulator and B-register each of 8 bits.
● Program counter – initializes from 00H(0d) to FFH(15d) during program execution.
● Memory Address Register (MAR) to store memory addresses.
● Adder/Subtracter for addition and subtraction instructions.
● A Control Unit
● A Simple Output.
● 6 machine reserved for each instruction

The instruction format of SAP-1 Computer is


the first four bits make the opcode while the last four bits make the address.

SAP-1 instruction set consists of following instructions

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

Machine cycle and Instruction cycle

SAP1 has six T-states (three fetch and three execute cycles) reserved for each instruction. Not all instructions require all the six T-states for execution. The unused T- state is marked as No Operation (NOP) cycle. Each T-state is called a machine cycle for SAP1. A ring counter is used to generate a T-state at every 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

Architecture of SAP-1

1. Program Counter (PC)
- it counts from 0000 to 1111 and it signals the memory address of next instruction to be fetch and executed.

2. Input and MAR (MAR)
- during a computer run, the address in the PC is latched in Memory Address Register (MAR).

3. RAM
- the program code to be executed and data for SAP-1 computer is stored here.
- during the a computer run, the RAM receives 4-bit addresses from MAR and a read operation is preformed. Also, the instruction or data word stored in RAM in placed on W bus for use by some other part of the computer.

4. Instruction Register (IR)
- IR contains the instruction, composed of OPCODE + ADDRESS, to be executed by SAP-1 computer.

5. Controller Sequencer-
- it generates the control signals for each block so that action occur in desired sequence. CLK signal is used to synchronize the overall operation of the SAP-1 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.

6. 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.

7. Adder-Subtracter
- it is a 2's complement adder-subtractor
- this module is asynchronous, which means that its contents can change as soon as the input word change.

8. B Register
- it is 8-bit buffer register which is primarily used to hold the other operand of mathematical operations.

9. Output Register
- this register hold the output of OUT instruction.

10. Binary Display
- it is a row of eight LEDs to show the contents of output register.

--to be able to have a better understanding of SAP-1's simulation, watch the video that i have post.

* This video was posted by Mark Anthony Jabson at www.youtube.com

After the discussion of SAP-1, the next thing is about early computer designs! I have posted an article coming from Marina del Rey @ www.hubpages.com

A Brief History of the Microcomputer 80

The "Chips" That Started it All Micro-Chip

All microcomputers use a processor called a mirco-chip. This is typically a small square sliver of silicon (glass) with pathways etched on it via a chemical process. These pathways can be transistors or simply "wires" that connect the transistors. If you can pack enough of these transistors on the silicon "die" then those transistors can become a processing unit, memory, and a calculator. The entire thing is called an "integrated circuit."

The First Microcomputer

Altair 8800

The Altair 8800, produced by Micro Instrumentation and Telemetry Systems (MITS), and invented by Ed Roberts, was originally sold as a kit. It had no operating system, but did recognize commands entered via a row of switches. The Altair had no keyboard and could not be connected to a television screen. The original intent of the Altair was as an offering to a wider range of hobbyists than the usual MITS customer. Up to this time MITS had been devising kits geared toward back-yard rocketry.

Radio Electronics magazine, at the time a competitor to Popular Electronics, was publishing a large number of "how to" articles on the building of digital multi-meters, voice transmitters and the like. Popular Electronics wanted to beat RE at their own game by being the first to publish a complete "how to" on a home computer. To do this Popular Electronics asked MITS to create a complete computer kit2, with all the parts included, along with a sleak and eye pleasing case. MITS complied with the RE-8, but Popular Electronics Alexander Burawa (associate editor), and John McVeigh (technical editor) came up with Altair as an alternative name.

The Altair was an idea presented to the public at just the right time. By working a quantity discount with Intel for $75 a unit for the CPU and the fact that a wealth of engineers had been required to take FORTRAN or BASIC in college, there was a ready market of potential buyers. Popular Electronics announced the kit in December 1974 for the January 19753 issue. By the end of May 1975 MITS had sold 5,000 Altairs.

A kit sold for $439 a fully assembled computer for $621.
In mid 1974 Intel, a transistor and memory chip manufacturer, came up with the first successful "computer on a chip" design called the 8080. It ran at two (2Mhz) MegaHertz and performed computing functions via an eight (8) bit1 Central Processing Unit (CPU). The 8080 had a forty (40) pin design that allowed for a sixteen (16) bit bus.

This was the improved version of the 8008 which had been designed for Computer Terminal Corporation (CTC). Because the 8008 did not meet CTCs performance specifications the 8080 was developed, but by then CTC was using their own chip. Intel found a home for the 8080 in the Altair (see below).
MOS Technology

In 1975 MOS Technology developed a similar chip to the 8080. The designer was Chuck Peddle of Motorola and his idea was to design a chip that was compatible to the Motorola 6800, but considerably cheaper. Initially there was little public interest because it was so cheap (at $25 compared to Intel's $179) that no one took it seriously. Fortunately, both Intel and Motorola did take it seriously, dropped their prices to under $80, and in the process made the 6502 attractive.

This was the first chip used in the Apple and Commodore computers.

1 A bit is the smallest unit of data measure on a computer. It represents a "switch" position of "on" or "off." Eight bits is a byte. A single character such as "A" takes up a byte.

Relating SAP-1 to early designs.

My Opinion: Architecture vs Design
 Architecture is the plan; design is the implementation
 Good architects understand design, good designers understand architecture

As i notice, early microcomputer designs are designed for a special purpose only. Some of them are for computing and some of them are for business purpose only, not like the SAP-1 Architecture, it can have lots of application. But as I studied, i saw some comparison of early design to the SAP-1 Architecture. One of them is the instruction cycle of the two. Both of them have the same instruction like LDA, ADD, SUB, OUT, HLT. They are also the same in some other features and attributes that complete the architecture.

I think that if i have enough time to study the SAP-1 architecture and read more about the early computer designs, I can have a lot of information about of them, especially in their similarities and differences. But as of now, this would be it. hehe!!

Hope you like my blog. I am so happy that Im done of my first ever BLOGGING