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Zilog Z80
Texas Instruments uses the Zilog Z80microprocessor in some of their calculators (TI-82,83,85,86,others?). That's how I got interested in the Z80 Microprocessor. Here's some info:

Around 1978 (or so), Intel introduced one of their first "real" cpu's, the 8080. Some dudes at Intel left and started their own company, Zilog. They created the Z80, wich has all the instructions of the 8080, and many more. The great advantage of the Z80 is that it can run 8080 programs.

Well, so much for the history of the Z80. Let's look at some stats: The Z80 is an 8 bit processor with a 16 bit address bus capable of direct access of 64k of memory space. It has a language of 252 root instructions and with the reserved 4 bytes as prefixes, acceses an additional 308 instructions. The Z80 was modeled after the 8080 and contains the 78 - 8080 opcodes as a subset to it's language.

Programming features include an accumulator and six eight bit registers that can be paired as 3-16 bit registers. In addition to the general registers, a stack-pointer, program-counter, and two index (memory pointers) registers are provided. While not in the same leauge as the 80486 or 68000 series, the Z80 is extremely useful for low cost control applications. One of the more useful features of the Z80 is the built-in refresh circuitry for ease of design with DRAMs.

The Z80 comes in a 40 pin DIP package. It has been manufactured in A, B, and C models, differing only in maximum clock speed. It also has been manufactured as a stand-alone microcontroler with various configurations of on-chip RAM and EPROM.

The Z80 in the TI-83 runs at a low 6 Mhz and is underclocked. According to the Z80 user Manual, the Z80 is capable of running at 20Mhz without exceeding the operating temperatures. So, it is possible to make your TI-83 run at 20 MHz. This isn't a smart thing to do though, the LCD driver wouldn't be able to handle all the instructions because it would still run at the same speed as it used to do, thus resulting in f*cked up display. It is possible to overclock your calc to 8 or 10 Mhz, but you might want to use a switch to switch between normal and boosted speed (overclocking drains your batteries faster than normal speed).

Yeah, I know. The picture says Toshiba, they manufactur Z80's too ;).
TI-83 CPU