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Emotional stories about processors for first computers: part 6 (TI TMS9900)

Texas Instruments TMS9900



I have never written codes for this very special processor, even though this is the first 16-bit processor available for use in personal computers. This processor was produced since 1976. It used a much rarer big-endian order of bytes. This order was used only in processors of the Motorola's 6800 and 68000 series and in the architecture of the gigantic IBM mainframes. All other processors in this review used little-endian byte order.

The TMS9900 has only three 16-bit registers: the program counter, the status register and the base register for pseudo-registers. This processor uses a dedicated 32-byte memory space (a context) as 16 double-byte registers. Such a way of use of memory is somewhat like the zero page memory in the 6502 architecture. Using the base register, the TMS9900 can very quickly change the context. This is similar to the Z80 which has two register contexts. Processor status flags are distinguished in originality, along with typical flags of carry, zero (equality), overflow, parity, there also are two more unique flags of logical and arithmetic less. Working with the stack and subroutines resembled the RISC-processors of the incoming future. There is simply no ready to use stack, but you can create a stack using one of the pseudo-registers. When a subroutine is called, a new value is selected for the counter and base registers, and all three registers are stored in the pseudo-registers of the new context. Thus calling a subroutine is more like calling a software interrupt. The TMS9900 has a built-in interrupt controller designed to work with hardware interrupts of up to 16.



The first 16-bit home computer – it has even color sprites!



The system of instructions looks very impressive, there are even multiplication and division. The unique X instruction allows you to execute one instruction on any memory address and move on to the next one. The execution of instructions is rather slow, the fastest instructions require 8 cycles and arithmetic instructions 14 cycles. However, multiplication (16*16=32) for 52 cycles and division (32/16=16,16) for only 124 cycles were probably the fastest among processors of the 70's.

The TMS9900 requires three supply voltages of -5, 5 and 12 volts and four phases of the clock signal – these are the worse specifications among the processors known to me. In 1979 this processor was demonstrated to IBM specialists, who then were looking for a processor for the IBM PC prototype. The obvious drawbacks of the TMS9900 (addressability of only 64 KB of memory, lack of the necessary controllers, relative slowness) made an appropriate impression and the Intel 8088 was chosen for the future leader among PC's. To deal with the lack of controllers Texas Instruments also produced the TMS9900 variant with an 8-bit bus, the TMS-9980, which worked 33% slower.

The TMS9900 used in the TI-99/4 and TI-99/4A computers which were fairly popular in the USA. They were "crushed" in the price war by the computer Commodore VIC-20 by 1983. Curiously as a result of this war Texas Instruments was forced to cut prices on its computer to the incredible price for 1983 of $49 (in 1979 the price was $1150!) and sold them with a big loss to themselves. As an example we can mention the relatively unpopular Commodore +4 computer, which ceased to be produced in 1986, the price of which fell to $49 in 1989 only. The TI-99/4A was stopped being produced in 1984, when because of the ultra-low prices it began to gain popularity. This computer might only be conditionally called 16-bit: only 256 bytes (very little) of its RAM and all of its ROM are addressable through a 16-bit bus. The rest of the memory and I/O-devices work over a slow 8-bit bus. Therefore it is possible to more correctly consider the BK0010 as the first 16-bit home computer. It is an interesting coincidence that the TI-99/4 and TI-99/4A use a processor at a frequency of 3 MHz – exactly the same as the BK0010 uses.

In the TI-99/4 and TI-99/4A a rather successful TMS9918 chip was used as a video controller, which became the basis for the very popular worldwide MSX standard, as well as for some other computers and game consoles. In the Japanese company Yamaha this video chip was significantly improved and was subsequently used in particular to upgrade the TI-99/4 and TI-99/4A themselves!

The TI-99/4 series is a rare example of computers where a processor and a computer manufacturer was the same.

Edited by Richard BN

Tags: 6502, 6800, 68000, 8088, be, commodore, computer, cpu, hardware, history, ibm pc, ibm/370, intel, le, motorola, processor, texas instruments, ti, tms-9900, vic-20
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