Page 10 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Introduction to Microprocessors
Notes
4 LOVELY PROFESSIONAL UNIVERSITY
function—keyboard, printer, display, serial arithmetic, control, etc. With only two designers,
Intel didn’t have the manpower to do that many custom chips. We needed to solve their problems
with fewer chip designs. Ted Hoff chose a programmed computer solution using only one complex
logic chip (CPU) and two memory chips; memory chips are repetitive and easier to design. Intel
was a memory chip company, so we found a way to solve our problem using memory chips!
In 1970 Intel designers implemented a 4-b computer on three LSI chips (CPU, ROM, RAM) housed
in 16-pin packages. Reducing the data word to 4-b (for a BCD digit) was a compromise between
1-b serial calculator chips and conventional 16-b computers.
Figure 1.2: Transistor
The scaled down 4-b word size made the CPU chip size practical (2200 transistors). We used the
16-pin package, because it was the only one available in our company. This limited pin count
forced us to time multiplex a 4-b bus. This small bus simplified the printed circuit board (PCB), as
it used fewer connections. However, the multiplex logic increased chip area of the specialized
ROM/RAM memory chips, which then had to have built-in address registers. Increasing the
transistor count to save chip connections was a novel idea. In school we learned to minimize
logic, not interconnections! Later, LSI “philosophers” would preach, “logic is free”.
1. MCS-4 Features:
256 × 8 Read Only Memory (2 kb ROM) with 4-b I/O port
80 × 4 Random Access Memory (320 b RAM) with 4-b output port
4-b CPU chip with: 16 x 4-b index registers, 45 1 and 2 byte instructions, 4-level
Subroutine Address Stack, 12-b Program Counter (4 k addresses).
(A) ROM Chip (4001)
Conventional calculators utilized specialized custom chips for keyboard, display, and printer
control. With the MCS-4 all control logic is done in firmware, program stored in ROM. A single
ROM chip design is customized (with a mask during chip manufacturing) for a customer’s
Figure 1.3: ROM Chip