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Emotional stories about processors for first computers: part 11 (Intel 8080)

Intel 8080 and 8085



The first real processor on a chip, made in the first half of 1974, is still being manufactured and is currently being used. It was repeatedly cloned around the world, in the USSR it had the designation KP580BM80A. Modern Intel processors for the PC still easily reveal their kinship to this in some sense relic product. I myself haven't written codes for this processor but being well acquainted with the architecture of the Z80, I would venture to give some of my comments.

The 8080 instruction system like other Intel processors for the PC can hardly be called ideal, but it is universal, quite flexible and has some very attractive features. The 8080 favorably differed from its competitors, the Motorola 6800 and the MOS Technology 6502, by a large number of even somewhat clumsy registers. The 8080 provided a user with one 8-bit accumulator, a 16-bit semi-accumulator and simultaneously fast index register HL, a 16-bit stack pointer, as well as two more 16-bit registers BC and DE. The BC, DE, and HL registers could also be used as 6 byte-registers. In addition the 8080 had support for an almost full set of status flags: carry, sign, zero and even parity and auxiliary carry. Some commands from the 8080 instruction set had been speed champions for a long time. For example the XCHG command makes the exchange of the contents of the 16-bit DE and HL registers in just 4 clock cycles, it was extremely fast! A number of other commands, although they did not set such bright records, were also among the best for a long time:


  • XTHL – exchange of HL register contents and data at the top of the stack, 18 cycles – it seems like a lot, but even on the real 16-bit 8086 an equivalent of such a command takes at least 26 cycles, and for the 6800 or 6502 such a command is hard to imagine;
  • DAD – add to the semi-accumulator HL the value of another 16-bit register (BC, DE or even SP), 10 cycles. This is a true 16-bit addition with a carry flag set. If you add HL to itself you will get a quick 16-bit shift left or multiplication by 2, which is a key operation for programming both full multiplication and division;
  • PUSH and POP – put in the stack and remove from the stack a 16-bit value respectively from a register or in a register. They perform in 11 and 10 cycles. These are the fastest 8080's operations for working with memory, and when they are executed SP is automatically incremented or decremented. The PUSH can be used for example to quickly fill memory with a pattern with values from 3 registers (BC, DE, HL). There are no stack instructions for working with 8-bit values at all;
  • LXI – a loading of a 16-bit constant into a register (HL, DE, BC, SP) for 10 cycles;
  • RNZ, RZ, RNC, RC, RPO, RPE, RP, RM – conditional returns from any subroutine, allow to make code cleaner eliminating the need to write extra conditional jumps. These commands were abandoned in the x86 architecture, but they should probably have been saved, the code with them turns out nicer.

This processor was used in the first 'almost personal computer' the Altair 8800, which became very popular after the journal publication in early 1975. By the way in the USSR a similar publication happened only in 1980 and corresponding to it in relevance only in 1986.



The first almost PC



The Intel's 8080 became the basis for the development of the first mass professional operating system CP/M, which occupied a dominant position among microcomputers for professional work until the mid-80's.

Now about the shortcomings. The 8080 required three supply voltages of -5, 5, and 12 volts. Working with interrupts was clumsy and slow. In general the 8080 was rather leisurely if you compare it with competitors which soon appeared. The 6502 could be up to 3 times faster when working on the same frequency as the 8080.

However in the architecture of the 8080 was laid as it turned out the correct vision of the future, namely it was a vision of a fact unknown in the 70's that processors would be faster than memory. The 8080's DE and BC registers are a prototype of modern caches with manual control, rather than general-purpose registers. The 8080 could use 2 MHz frequency, while competitors could only use 1 MHz, which reduced the performance difference between them.

It's hard to call the 8080 a 100% 8-bit processor. Indeed its ALU is 8 bits wide, but there are many 16-bit commands that work faster than if you use only 8-bit counterparts instead, and for some instructions there are no 8-bit analogs at all. The XCHG instruction is essentially and by timing 100% 16-bit and there are real 16-bit registers. Therefore I venture to call the 8080 partially 16-bit. It would be interesting to calculate this processor's bit index based on the set of its features, but as far as the author knows, no one has still done such work.

The author of this text does not know the reasons why Intel abandoned direct support of the 8-bit PC's with their processors. Intel has always distinguished the complexity and ambiguity of the policy. Its connection with politics in particular is illustrated by the fact that for a long time Intel has had fabs in Israel and until the end of the 90's it was secret. Intel practically did not try to improve the 8080, only the clock frequency was raised to 3 MHz. In fact the 8-bit computer market was given to Zilog with the z80 processor which was related to the 8080, and the z80 was able to quite successfully withstand the main competitor, The Terminator 6502.

In the USSR and Russia the domestic clone of the 8080 became the basis of many popular computers that remained popular until the early 90s. Those are of course the Radio-86RK, Mikrosha, the multicolor Orion-128, Vector, and Corvette. Eventually cheap and improved ZX Spectrum clones based on the z80 won the clone wars.




This is a real PC



In early 1976 Intel introduced the 8085 processor, compatible with the 8080, but significantly superior to its predecessor. In it the power supply of -5 and 12 volts has become unnecessary and the connection scheme has been simplified, work with interrupts has been improved, the clock frequency has been used from 3 to a very solid 6 MHz, the command system has been expanded with very useful instructions: 16-bit subtraction, 16-bit shift right for only 7 cycles (it was very fast), 16-bit rotate left through the carry flag, loading of a 16-bit register with an 8-bit offset (this instruction is possible to use with the stack pointer too), writing of the HL register contents to an address in the DE register, analogous reading of the HL via an address in the DE. All the instructions mentioned above, except for the shift to the right, are executed in 10 cycles – this is sometimes significantly faster than their counterparts or emulation on the Z80. Some more instructions and even two new processor status flags were added. Among the new flags it is worth noting the overflow flag, although the work with it was almost not supported. In addition many instructions for working with byte data were accelerated by 1 clock cycle. This was very significant as many systems with the 8080 or Z80 used wait states, which due to the presence of extra cycles on the 8080 could stretch the execution time by almost twice. For example in the mentioned computer Vector, register-register instructions were performed for 8 cycles, and if there were the 8085 or Z80, then the same instructions would be executed only in 4 cycles. The XTHL instruction became faster even by two cycles. With the new instructions you can write code to copy a block of memory that runs faster than the Z80's LDI/LDD commands! However, some instructions, for example a 16-bit increment and decrement, the PUSH and conditional returns became slower by a cycle.

The 8085 has built-in support for interrupts, which in many cases eliminates the need for a separate interrupt controller in a system, and a serial I/O port. As already noted in the 8085 the full support was not added for the overflow flag, so the arithmetic of signed numbers remained somewhat incomplete.

However I can repeat the statement "for unknown reasons" Intel refused to promote the 8085 as the main processor for PC's. It was only in the 80's that some fairly successful 8085-based systems appeared. The IBM System/23 Datamaster first appeared in the 1981, it was a predecessor and almost a competitor to the IBM PC. Then in 1982 a very fast computer with excellent graphics, the Zenith Z-100, was released, in which the 8085 was running at 5 MHz. In 1983 Japanese company Kyotronic created a very successful KC-85 laptop, versions of which were also produced by other companies: Tandy were producing the TRS-80 model 100, NEC – the PC-8201a, Olivetti – the M-10. In total they released perhaps more than 10 million of these computers! In Russia in the early 90's on the basis of domestic clone the ИM1821BM85A there were attempts to improve some systems, for example, the computer Vector. Surprisingly the main processor of the Sojourner rover, which reached the surface of Mars in 1997, was the 8085 at 2 MHz!

In fact Intel gave way to the Z80 in the 70's. A few years later in the battle for the 16-bit market Intel behaved quite differently, starting a lawsuit to ban sales of the V20 and V30 processors in the United States. Interestingly the mentioned processors of Japanese company NEC could switch to full binary compatibility with the 8080, which made them the fastest processors of the 8080 architecture.

Another secret from Intel is the refusal to publish an extended command system, including support for two new flags. However one of the official manufacturers of these processors has published the entire system of instructions. What are the reasons for this strange refusal? One can only guess. Could Zilog then have played a role, that AMD might have once played, and created the ostensible appearance of competition while the 8085 could have brought down Zilog? Was it maybe about wanting to keep the system of instructions closer to the 8086 then being designed? The latter seems doubtful. The 8086 was released more than 2 years after the release of the 8085 and it’s hard to believe that in 1975 the system of its commands was already known. And in any case compatibility with both the 8080 and 8085 on the 8086 is achievable only with the use of a macro processor, sometimes replacing one of the 8080's or 8085's instruction with several of its own. Moreover the two published new instructions of the 8085 in the 8086 are not implementable at all. It is especially difficult to explain why Intel did not publish information about new instructions after the release of the 8086. We can also assume that most likely it was in the marketing. Due to artificially worsening specifications of the 8085, they received on this background a more spectacular 8086.

Edited by Richard BN

Tags: 6502, 6800, 8080, 8085, 8086, altair 8800, computer, cpu, hardware, history, intel, mos technology, motorola, pc, processor, x86, z80, zilog
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