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8085 Information

by Andrew Diller 1997

The 8085 is the CPU for the whole line of the Kyocera family of computers, which of course includes the Model 100/102/200. While reading a posting about CP/M in folklore.computers, I asked Mr. Congdon about the possibility of CP/M for the M100. I have never heard of it being available for the Tandys. I also was interested in the CPU itself. As you will read for yourself, Mr. Congdon sent me a very informative reply which included some good information about the Brain of the Model 100
If you have any info about this subject, please send it my way!

Andrew Diller,Sat, 06 Jan 1996


>Speaking of the 8085, it is the brain of the Tandy Model 100. Do you or any one else know if CPM was written for the m100?

Not that I know of. I'm sure CP/M could have been adapted to the 100 if it did indeed have all the required features. There was a disk controller for the 100 although I don't know if many people ever bought them. As I recall, the 100 disk drive system added some additional commands to the built in BASIC and also extended the save/load commands of the other built in programs. The review I read of the disk drive system was rather negative.

>Also, isn't the 8085 an 8086 that 'only' draws 5 volts??
>-OR- why did intel make the 8085??

The answer to the first question is YES. The 8085 required only a single +5V power supply as opposed to the 8080 which required three (+5V, +12V, and -5V). Intel made the 8085 as a significant improvement on the 8080. Basically they preserved full 8080 compatibility but improved the hardware so designers could implement the processor in a simpler manner. The key improvements to the 8085 were:

Single +5V power supply
Internal clock generator
Internal status line decoder
Improved interrupt capability
Several new instructions

The internal clock generator allowed one to drive the 8085 with just an external crystal and two capacitors. This was MUCH simpler than the 8080 which required a non-TTL (+11V) non-symmetrical clock. On an 8080 this was accomplished with an external IC which created the right clock.

The status line decoder was also an external IC on the 8080. This IC broke out the RD, WR, MEM, IO, and some other control lines. On the 8085 all these functions had their own pins.

The 8085 had more types of interrupts possible (3 modes I think) and also some improvement to the software callable interrupt mode.

Several additional instructions were added to supplement the existing 8080 instruction set. The two most important were RIM and SIM which controlled the interrupt modes and also allowed two external pins to be read and written. Using RIM and SIM along with the two pins (called SID and SOD), you could implement a serial port all in software!

I don't know if Intel brought out the 8085 in answer to Zilog's Z80 or not. The Z80 was Zilog's idea of an improved 8080 as the 8085 was Intel's.

Why did the development of the intel/ms-dos line follow the 8086->8088 ->80286 ->386 -> 486 -> 586 ...etc- effectively leaving the 8085 as a dead in the tree?

Is that just the way it happened? What changed in the 286 to allow it to be 8088 compatible, but the 8085 was not?

What I am trying to say might be a little jumbled, but I think I can sum it up by asking what happened to the 8085? Was it a test bed for the 286? Did it not generate enough interest in manufacturs?

I think I can clear this up. You've gotten a few parts mixed. Here is the baseline of Intel's processors arranged chronologically.

4004->4040->8008->8080->8085->8086->8088->80186->80286->80386->80486-> Pentium->Pentium Pro

The 4004 and 4040 were 4-bit processors and Intel's first venture into microprocessors. They were used in calculators and other low-end devices.

The 8008 was Intel's first 8-bit processor but was rather limited so it wasn't used much in general-purpose computers. It was mainly used to give some intelligence to hardware. One application was a serial terminal where the 8008 coordinated the terminal's activities.

The 8080 was Intel's first useful 8-bit processor. It was quite capable and was ultimately used in MANY early personal computers.

The 8085 was simply an improved 8080 with the features which I've already mentioned. Note that it was still an 8-bit processor.

The 8086 was Intel's first venture in 16-bit computing. It had a totally different instruction set, different hardware architecture, and much greater capability. It was in no direct way related to ANY of the previous devices (including the 8085). It just happened to be the next number in sequence.

The 8088 was an 8086 internally with an 8-bit data bus. It WAS a 16-bit processor but had an 8-bit external bus to make connection to older hardware less difficult. It also had no direct relation to previous processors. The 8088 was used in the IBM PC and XT and most XT-class clones.

The 80186 was an enhanced 8086 with some added instructions. There was also an 80188. Neither were used much in PC-type machines. The only example I am aware of is the Tandy 2000 which used it. The 80186 and 80188 were often found in intelligent peripherals like disk controllers.

The 80286 was the next major jump in the 80x86 line. It had full 8086 compatibility but also included a protected mode which expanded the addressing to allow more memory to be accessed. It also had multitasking protection built in. The instruction set was expanded to include some additional sophisticated 16-bit instructions.

The 80386 was the Intel's first popular 32-bit processor. It included full 8086 and 80286 compatibility but added some extra modes including a software switchable protected mode and a virtual machine mode. It had larger addressing and also 32-bit instructions.

The 80486 was an enhanced 386 with an internal cache and math processor (except in the SX models). I think there are some extra instructions as well. Various models of the 486 exist including a number of clock doubler and tripler models (DX2 and DX4).

The Pentium and Pentium Pro are 64-bit processors with full backwards compatibility but MANY new features including some RISC-like features and some superscalar capabilities. They include caches, pipelining, anticipatory fetching, predictive branching, and many other things. The Pentium Pro is the new P6 processor which includes a massive internal cache and many more advanced RISC-type features which supersede those found in the plain Pentium.

Anyway, as you can see, the 8086 and 8085 have no connection and are not members of the same family. The 8086 was a complete departure from the previous 8080 family and a major step-up for Intel. The 8085 was relatively popular in 8080-type applications and was used in a number of personal computers. I think the reason you don't hear much about it is due to the introduction of Zilog's Z80 which came at about the same general time. The Z80 was regarded by many as the superior processor and was eventually the 8-bit processor of choice. If the Z80 had not come out, I suspect the 8085 would have found its way into many more designs.

I hope the above summary is helpful. I don't promise that everything is perfectly accurate as I am just doing this on the fly. There are many other things which I have not mentioned especially relating to the newer Intel devices like the Pentium Pro. It would take PAGES to relate all its features. The summary I have provided is just to give you a feel for the general chronology of Intel's most popular processors.

A good source of technical data for the 8085 would be Intel's older databooks from the late 1970's or early 1980's. I'm sure they don't publish data now although you CAN buy the chip still and it is still used in some controller designs. Look in a good library for older data books and you will find plenty of data.

Another source which I found helpful was the book "Microprocessor Handbook" which I think is authored by someone named Greenfield or Greenberg. I don't have copy here so the information is coming from memory. It contains chapters about all the major 8 and early 16-bit processors. It is excellent!

Don Congdon

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