Jonathan A. Titus

Microcomputer Pioneer

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The mark-8 consisted of 6 different boards. Below are the discriptions of the boards, their tasks, and serveral images of the boards.

CPU Board
The Central Processing Unit (CPU) board contains the microprocessor IC and extra circuitry that is interfaced with the rest of the computer. The 8008 microprocessor was fabricated as an MOS circuit and the outputs only drive one low-power circuit. The outputs are buffered with an inverter before being used.

The Mark-8 computer is controlled by the 2-phase clock supplied by a crystal oscillator. This oscillator controls the pulse widths and frequency. The logical operations of the cumputer circuts are controled by the clock and synchronization signal supplied by the microprocessor. The microprocessor has 3 state-output signals S0,S1, and S2 which are used to drive the decoder. The eight possible states are then used to conrol other functions in the interface logic.

Register Display
This LED Register Display board provides a visual indication of the HI and LO address latches and the memory data in the selected memory locations indicated by that address. There is also a readout for the output port 0. For programming this visual output can be key to represent eacy byte of data. Each of the output registers are represented by the 8 LED indicators, 1 ON, 0 OFF. As the data is changing the indicators will change with them. To send the data to this ouput port 0 use the OUT instruction 01010001 or 1218.

Output Ports
This output ports board is used to send the data from the computer to external devices, such as the teletype. Four output latches are provided on this board and two of these may be used with the Mark-8. The second may only use three of the output latches.

Memory Latch
The Memory Latch uses 1101 type semiconductor, integrated circuit memory. The 1101 random access memory (RAM) is organized as a 256 x 1-bit memory. This allows for 256, 8 bit words. The minimum amount of memory required for the Mark-8 is 256. Each of them memory modules can store up to 1k of memory. Upto 4 memory modules may be used on the Mark-8 allowing for a maximum of 4k of memory. At the time it was more than enough to run most applications.

Address Latch
The address latch module is used to hold the data which is to be used as the memory address. There are two 8-bit latches that are provided since the computer will use onset of eight bits for memory address the toher set is for the control functions. This microprocessor can directly address up to 16.424 words of memory (16K). This means that we will need 14 binary bits for the complete address of the memory.

Input Multiplexer
The input multiplexer is used to control all of the data that is inputed into the Mark-8. All data going into the computer is placed on the I/O bus during the IN cycle signalled using the IN signal. Since data can be coming in from a number of different experiments or sources, there are the means to selecting which of the data is sent to the CPU. This module allows a user to connect an ASCII keyboard to give the Mark-8 more usability.