Page 15 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Unit 1: Microprocessors and Microcomputers

Notes
Figure 1.9: 8080 Chip

Deleting the on-chip stack saved chip area, but was a net advantage to the user—now the stack
had unlimited size. In the 8080, the registers were arranged as pairs of 8 b, to provide 16-b data
handling. The three register pairs were designated as: BC, DE, HL. The High/Low register pair
was the only way to address memory in the older 8008. This was limiting to programmers, so in
the 8080 direct memory addressing instructions were added, as well as several specialized
instructions for the HL register pair. One instruction, XTHL, provided for exchanging the top of
stack with HL; another instruction, XHLD, swapped the contents of HL with the DE register pair.
As these special instructions were not very symmetric, applying only to HL, we optimized their
logic implementation. One of Ted Hoff’s tricks was the use of an exchange flip-flop for DE/HL.
This flip-flop designated one of the pairs as HL and the other register pair as DE. Simply toggling
this flip-flop affected an apparent exchange! This saved a lot of logic; but by mistake, the reset pin
had been connected to this flip-flop. An early 8080 user manual stated: “after reset, the HL/DE
register contents may be exchanged” (later the reset connection was cut). The lack of instruction
set symmetry was a nuisance to programmers and later CPU’s instruction sets were considerably
more regular; of course there were more transistors “to bum.”
(B) No More

This is why the last 12 instructions were never implemented and why there was room in the
instructions set for the 8085 microprocessor’s added instructions.The 8080 was very successful in
the market. Meanwhile, competition blossomed and a variety of great processors developed
including the Motorola 6800 and the MOS Technology 6502. The 8080 CPU chip was patented by
Intel and has three claims.
(C) 8085

To meet competition in 1976, Intel decided to develop a more integrated version of the 8080. This
chip contained 6500 transistors. The new N-MOS was more TTL compatible and this chip needed
few external parts.
There were 12 unused operation codes in the 8080 which provided room to expand the CPU’s
function. At Intel, a committee studied, argued, and finally decided after many months which
instructions to add. Although all of these new codes were utilized by the 8085 designers, by the
time this product got to market it was almost obsolete. To reduce compatibility requirements with
the 8086 which was in design, 10 of the new 12 instructions were never announced in the data sheet.
They have only been an interesting historical anomaly and a lesson about design by committee.

Figure 1.10: 8085 Chip

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