IBm PC/AT - Mark Dean

The John Fisher Distinguished Professor in the Department of Electrical Engineering and Computer Science at the University of Tennessee

Mark Dean was born in Jefferson City, Tennessee on March 2, 1957. A bright and energetic child, he often endured questions from grade school classmates, asking if he was really Black because Black people were not supposed to be that smart. Mark was an outstanding high school athlete as well as a straight A student. His success continued in college as he graduated at the top of his class with a Bachelor of Science in Electrical Engineering from the University of Tennessee in 1979.

The Birth of the IBM- PC

At the dawn of the 1980s the then new personal computer industry, was dominated by the Commodore PET, Atari, Apple II, and Tandy Corporation's TRS-80. With $150 million in sales by 1979 and projected annual growth of more than 40% in the early 1980s, the microcomputer market was large enough for IBM's attention. Whether IBM had waited too long to enter an industry in which Apple and others were already successful was unclear.

Then IBM President John Opel (1974-85) was not among those skeptical of personal computers (PCs). He put executive William Lowe was in charge of developing a PC for IBM. Lowe proposed that the new computer have an open architecture, use non-proprietary components and software, and be sold through retail stores, all contrary to standard IBM practice.

Lowe got promoted.

Don Estridge became head of PC project. His key team members included Bill Sydnes, Lewis Eggebrecht, David Bradley, Mark Dean, and David O'Connor.

Mark Dean would be granted  three of the original nine patents that all PCs are based upon.

The IBM Personal Computer, commonly known as the IBM-PC, is the original version and progenitor of PC compatible machines. It was introduced on August 12, 1981.


IBM Personal Computer / AT

The IBM Personal Computer / AT (Type 5170) was presented to the public on August 14, 1984.


The AT used as a CPU (central processing unit) what was then the latest Intel processor (80286,) at initially 6 MHz and later 8 MHz…


The hertz (symbol Hz) is the unit of frequency. It is defined as one cycle per second.. It is used to describe the speeds at which computers and other electronics are driven. Hertz are commonly expressed in multiples kilohertz (103 Hz, symbol kHz), megahertz (106 Hz, MHz), gigahertz (109 Hz, GHz), and terahertz (1012 Hz, THz). Kilo equals one thousand, mega equals one million, giga equals one billion and tera equals one trillion.


…which made possible through the new protected mode of the CPU, a memory expansion to a 20MB hard drive.


A new feature was the 16-bit wide system bus in place of the previous 8-bit bus. The AT extended the slot by just a few contacts, and the new bus remained compatible with the previous 8-bit cards to be operated as well. This bus was developed by a team led by Mark Dean.


Get on the Bus


In a computer, a bus is a communication system that transfers data between components inside a computer, or between computers. Computer systems generally consist of three main parts: a CPU that processes data, memory that holds the programs and data, and I/O (input/output) devices such as the computer’s screen, a mouse and keyboard. Buses move data between all of these devices.

This expression covers all related hardware components (wire, optical fiber, etc.) and software, including communication protocols. An attribute generally used to characterize a bus is that power is provided by the bus for the connected hardware.


In most traditional computer architectures, the CPU and main memory tend to be tightly coupled. In most cases, the CPU and memory share signaling characteristics and operate in conjunction. The bus connecting the CPU and memory is often referred to simply as the system bus. An external bus, or expansion bus, is made up of the electronic pathways that connect the different external devices, such as printer etc., to the computer.

Early microcomputer bus systems were such that each pin of each connector is linked to the same relative pin of all the other connectors). It was possible to allow peripherals to communicate with memory by attaching adaptors in the form of expansion cards directly to the system bus. As the number of potential peripherals grew, using an expansion card for every peripheral became increasingly untenable. This has led to the introduction of bus systems designed specifically to support multiple peripherals. However, these high-performance systems are generally too expensive to implement in low-end devices, like a mouse. This has led to the parallel development of a number of low-performance bus systems for these solutions, the most common example being the standardized Universal Serial Bus (USB).


Industry Standard Architecture:


Industry Standard Architecture (ISA) became the term for the 16-bit internal bus of IBM PC/AT and similar computers based on the Intel 80286. This bus design was (largely) backward compatible with the older IBM PCs as well as IBM PC compatibles.

In 1984, Dean and team member Dennis Moeller filed U.S. Patent request No. 4,528,626


Microcomputer system with Bus Control means for Peripheral Processing devices


Patent number: 4528626


Abstract: A microcomputer system includes a main processor, a memory and a direct memory access controller (DMA) effective to control direct data transfer between the memory and input/output devices on channels. Bus control for data transfer is switchable between the DMA and processor[s] by a hold request/acknowledge handshaking [the action of exchanging standardized signals between devices in a computer network to regulate the transfer of data] sequence between the DMA and processor. A control line from the channels is activated by a peripheral processing device on a channel when it wishes to gain control of the busses for data transfer. Logic means co-act with the handshaking sequence to determine which device gains control of the busses. This logic is responsive to the DMA address enable output (AEN), the hold acknowledge output of the main processor (HLDA) and the channel control line output (-MASTER). When all these are deactivated, control passes to the main processor, when AEN and HLDA only are activated, control passes to the DMA controller and, when all three are activated, control passes to the peripheral processing device.


Type: Grant                                             Filed: March 19, 1984


Date of Patent: July 9, 1985        Assignee: International Business Machines Corporation


Inventors: Mark E. Dean, Dennis L. Moeller


In 1985 they were granted the patent.


The ISA lasted until 1993 when it was replaced by more modern systems.




Ohm's law there are three fundamental components that stand in the way. The first is current, this is the movement of electrons in the circuit. Next there is voltage, the pressure of the electrons applied to the circuit. Lastly, there is resistance, which is a measure of the opposition an object gives to current passing through it. Now for Ohm's law,


I = V/R


where I = current in amperes,            V = voltage in volts, and         R = resistance in ohms.


Basically, this says that the current is proportional to the voltage divided by the resistance.

For any questions, please contact David Trotman :


1519 O'farrell St. San Francisco, CA 94115