Page 14 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Introduction to Microprocessors
Notes
8 LOVELY PROFESSIONAL UNIVERSITY
address registers, multiplexors and I/0 latch chips would need to be added to make the system
work; in practice about 40 additional small chips were needed. But standard memories were
available in high volume at low cost, and in a larger system the extra chip overhead could be
tolerated. Using memory chips with different access times requires a synchronizer scheme, and
therefore ready/wait signal pins were provided to perform a handshake function. These interface
signals are more sophisticated in today’s processors, but the 8008 demonstrated the idea.
The availability of Electrically Programmed ROM’s (EPROM) was significant in allowing
customers to experiment with their software. A product synergy evolved between Intel’s memory
component business and the microprocessor. Intel had an 18-pin package in volume production
for the 1k dynamic RAM chip (1103); this gave two more pins for the 8008 than we had on the
MCS-4, but we still had to time-multiplex an 8-b bus. By reducing the Program Counter width to
14-b we saved two package pins. The jump instruction contained a 16-b address, but two of the
bits were ignored. The 8008 could have 16k bytes of memory, and at the time, this seemed
enormous. (Today, users want 16 meg.)
(A) Little Endian
Some have wondered why the addresses in the 8008 were stored “backward” with the little end
first, e.g. the low order byte of a two-byte address is stored in the lower addressed memory
location. We (regrettably) specified this ordering as part of the JUMP instruction format in the
spirit of compatibility with the Data point 2200. Recall that their original processor was bit serial;
the addresses would be stored low to high bit in the machine code (bit-backward). Other computer
makers organize the addresses with the “big end” first. The lack of standardization has been a
problem in the industry.
(B) Applications
One of the first users of the 8008 was Seiko in Japan for a sophisticated scientific calculator. Other
users included business machines and a variety of general purpose computers.
Intel did not apply for a patent on the 8008. Data point contracted with Texas Instruments in 1970
to get a second source for this chip. TI patented their design, but never got into production.
After about one year of experience with programming the 8008 CPU chip, we had a number of
requested enhancements from our users. We proposed to build the 8080 as a follow on chip; this
chip was very popular and led to the microcomputer revolution and the Personal Computer. It is
ironic that Data point ultimately competed in the marketplace with PC products based upon
their own, Data point defined, architecture!
1.2.4 8080 More and No More
(A) More
Based upon Intel’s success with their new microcomputer product line Faggin convinced Vasdasz
in 1972 to fund a project to convert the P-MOS 8008 into the newer N-MOS technology. This
technology offered about a 2x speedup without making logic changes. After a short study, it was
determined that a new mask set was needed because of the incompatibility of transistor size
ratios. Faggin reckoned that since a new mask set was needed, he would fix some of the 8008’s
shortcomings.
We evolved the 8080 specification to improve performance 10x. We used the greater density to
put in more logic (4500 transistors) and do more in parallel; the on chip control logic grew by
50%.
We put the stack in memory, did 16-b operations, and improved memory addressing. Now 40-
pin plastic packages were available, and the address bus and data bus could be brought out in
parallel. This design also simplified the external circuitry and TTL voltage compatible signals
were provided.