Introduced in 1976 and based on the
Texas InstrumentsTexas Instruments , widely known as TI, is an American company based in Dallas, Texas, United States, renowned for developing and commercializing semiconductor and computer technology. TI is the No. 4 manufacturer of semiconductors worldwide after Intel, Samsung and Toshiba, and is the top supplier...
990The TI-990 was a series of 16-bit minicomputers sold by Texas Instruments in the 1970s and 1980s. The TI-990 was a replacement for TI's earlier minicomputer systems, the TI-960 and the TI-980...
minicomputerA minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
CPUThe Central Processing Unit or processor is the portion of a computer system that carries out the instructions of a computer program, and is the primary element carrying out the computer's functions. This term has been in use in the computer industry at least since the early 1960s...
, the
TMS9900 was one of the first true
16-bit-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor.Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816...
microprocessors (the first were probably
National SemiconductorNational Semiconductor is a semiconductor manufacturer, specializing in analog devices and subsystems,headquartered in Santa Clara, California, USA. The products of National Semiconductor include power management circuits, display drivers, audio and operational amplifiers, communication interface...
IMP-16The IMP-16, by National Semiconductor, was the first multi-chip 16-bit microprocessor. It consisted of five PMOS integrated circuits: four four-bit RALU chips providing the data path, and one CROM providing control sequencing and microcode storage.The IMP-16 provided four 16-bit accumulators,...
or
AMD-2901Am2900 is a family of integrated circuits created in 1975 by Advanced Micro Devices . They were constructed with bipolar devices, in a bit-slice topology, and were designed to be used as modular components each representing a different aspect of a computer control unit...
bit slice processorBit slicing is a technique for constructing a processor from modules of smaller bit width. Each of these components processes one bit field or "slice" of an operand. The grouped processing components would then have the capability to process the chosen full word-length of a particular software design...
s in 16 bit configuration). It was designed as a single chip version of the
TI 990The TI-990 was a series of 16-bit minicomputers sold by Texas Instruments in the 1970s and 1980s. The TI-990 was a replacement for TI's earlier minicomputer systems, the TI-960 and the TI-980...
minicomputerA minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
series, much like the
IntersilIntersil Corporation specializes in the design and manufacture of high-performance analog semiconductors for four high-growth markets — Communications, Computing, High End Consumer and Industrial.-Company history:...
6100 was a single chip
PDP-8The PDP-8 was the first successful commercial minicomputer, produced by Digital Equipment Corporation in the 1960s. DEC introduced it on 22 March 1965, and sold more than 50,000 systems, the most of any computer up to that date...
, and the
FairchildFairchild was an aircraft and aerospace manufacturing company based at various times in Farmingdale, New York; Hagerstown, Maryland; and San Antonio, Texas.-Early aircraft:...
9440 and
Data GeneralData General was one of the first minicomputer firms from the late 1960s. Three of the four founders were former employees of Digital Equipment Corporation. Their first product, the Nova, was a 16-bit minicomputer. This was followed by the Supernova and Eclipse product lines, all of which were...
mN601 were both one chip versions of
Data GeneralData General was one of the first minicomputer firms from the late 1960s. Three of the four founders were former employees of Digital Equipment Corporation. Their first product, the Nova, was a 16-bit minicomputer. This was followed by the Supernova and Eclipse product lines, all of which were...
's
NovaThe Data General Nova was a popular 16-bit minicomputer built by the United States company Data General starting in 1969. The Nova was packaged into a single rack mount case and had enough power to do most simple computing tasks. The Nova became popular in science laboratories around the world, and...
. Unlike the IMS 6100, however, the TMS 9900 had a mature, well thought out design.
It had a 15-bit
address busAn address bus is a computer bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus...
, a 16-bit
data busIn computer architecture, a bus is a subsystem that transfers data between computer components inside a computer or between computers.Early computer buses were literally parallel electrical buses with multiple connections, but the term is now used for any physical arrangement that provides the...
, and three internal 16-bit
registerIn computer architecture, a processor register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage available elsewhere. Most, but not all, modern computers adopt the so-called load-store architecture...
s (
PCThe program counter, or PC is a processor register that indicates where the computer is in its instruction sequence...
, WP, and
STA status register is a collection of flag bits for a processor. A popular example of a status register is the FLAGS register of x86 architecture based microprocessors....
).
Introduced in 1976 and based on the
Texas InstrumentsTexas Instruments , widely known as TI, is an American company based in Dallas, Texas, United States, renowned for developing and commercializing semiconductor and computer technology. TI is the No. 4 manufacturer of semiconductors worldwide after Intel, Samsung and Toshiba, and is the top supplier...
990The TI-990 was a series of 16-bit minicomputers sold by Texas Instruments in the 1970s and 1980s. The TI-990 was a replacement for TI's earlier minicomputer systems, the TI-960 and the TI-980...
minicomputerA minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
CPUThe Central Processing Unit or processor is the portion of a computer system that carries out the instructions of a computer program, and is the primary element carrying out the computer's functions. This term has been in use in the computer industry at least since the early 1960s...
, the
TMS9900 was one of the first true
16-bit-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor.Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816...
microprocessors (the first were probably
National SemiconductorNational Semiconductor is a semiconductor manufacturer, specializing in analog devices and subsystems,headquartered in Santa Clara, California, USA. The products of National Semiconductor include power management circuits, display drivers, audio and operational amplifiers, communication interface...
IMP-16The IMP-16, by National Semiconductor, was the first multi-chip 16-bit microprocessor. It consisted of five PMOS integrated circuits: four four-bit RALU chips providing the data path, and one CROM providing control sequencing and microcode storage.The IMP-16 provided four 16-bit accumulators,...
or
AMD-2901Am2900 is a family of integrated circuits created in 1975 by Advanced Micro Devices . They were constructed with bipolar devices, in a bit-slice topology, and were designed to be used as modular components each representing a different aspect of a computer control unit...
bit slice processorBit slicing is a technique for constructing a processor from modules of smaller bit width. Each of these components processes one bit field or "slice" of an operand. The grouped processing components would then have the capability to process the chosen full word-length of a particular software design...
s in 16 bit configuration). It was designed as a single chip version of the
TI 990The TI-990 was a series of 16-bit minicomputers sold by Texas Instruments in the 1970s and 1980s. The TI-990 was a replacement for TI's earlier minicomputer systems, the TI-960 and the TI-980...
minicomputerA minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
series, much like the
IntersilIntersil Corporation specializes in the design and manufacture of high-performance analog semiconductors for four high-growth markets — Communications, Computing, High End Consumer and Industrial.-Company history:...
6100 was a single chip
PDP-8The PDP-8 was the first successful commercial minicomputer, produced by Digital Equipment Corporation in the 1960s. DEC introduced it on 22 March 1965, and sold more than 50,000 systems, the most of any computer up to that date...
, and the
FairchildFairchild was an aircraft and aerospace manufacturing company based at various times in Farmingdale, New York; Hagerstown, Maryland; and San Antonio, Texas.-Early aircraft:...
9440 and
Data GeneralData General was one of the first minicomputer firms from the late 1960s. Three of the four founders were former employees of Digital Equipment Corporation. Their first product, the Nova, was a 16-bit minicomputer. This was followed by the Supernova and Eclipse product lines, all of which were...
mN601 were both one chip versions of
Data GeneralData General was one of the first minicomputer firms from the late 1960s. Three of the four founders were former employees of Digital Equipment Corporation. Their first product, the Nova, was a 16-bit minicomputer. This was followed by the Supernova and Eclipse product lines, all of which were...
's
NovaThe Data General Nova was a popular 16-bit minicomputer built by the United States company Data General starting in 1969. The Nova was packaged into a single rack mount case and had enough power to do most simple computing tasks. The Nova became popular in science laboratories around the world, and...
. Unlike the IMS 6100, however, the TMS 9900 had a mature, well thought out design.
It had a 15-bit
address busAn address bus is a computer bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus...
, a 16-bit
data busIn computer architecture, a bus is a subsystem that transfers data between computer components inside a computer or between computers.Early computer buses were literally parallel electrical buses with multiple connections, but the term is now used for any physical arrangement that provides the...
, and three internal 16-bit
registerIn computer architecture, a processor register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage available elsewhere. Most, but not all, modern computers adopt the so-called load-store architecture...
s (
PCThe program counter, or PC is a processor register that indicates where the computer is in its instruction sequence...
, WP, and
STA status register is a collection of flag bits for a processor. A popular example of a status register is the FLAGS register of x86 architecture based microprocessors....
). One unique feature, though, was that all general purpose user registers were actually kept in external memory. A single workspace register (WP) pointed to the 16 register set (each register being 16 bits wide) in RAM, so when a
subroutineIn computer science, a subroutine or subprogram is a portion of code within a larger program, which performs a specific task and is relatively independent of the remaining code....
was entered or an
interruptIn computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
was processed, only the single workspace register had to be changed - unlike some CPUs which required dozens or more register saves before acknowledging a
context switchA context switch is the computing process of storing and restoring the state of a CPU such that multiple processes can share a single CPU resource. The context switch is an essential feature of a multitasking operating system. Context switches are usually computationally intensive and much of the...
.
This was feasible at the time because RAM was often faster than the
CPUsThe Central Processing Unit or processor is the portion of a computer system that carries out the instructions of a computer program, and is the primary element carrying out the computer's functions. This term has been in use in the computer industry at least since the early 1960s...
. A few modern designs, such as the
INMOSInmos Limited was a British semiconductor company, founded by Iann Barron, based in Bristol and incorporated in November 1978.- Products :...
Transputers, use this same design using caches or rotating buffers, for the same reason of improved context switches. Other chips of the time, such as the 65xx series had a similar philosophy, using index registers, but the TMS 9900 went the farthest in this direction.
That wasn't the only positive feature of the chip. It had flexible interrupt-handling features, and a simple but powerful instruction set. 15 of the 16
registerIn computer architecture, a processor register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage available elsewhere. Most, but not all, modern computers adopt the so-called load-store architecture...
s could be indexed although it should be remembered these registers were actually in memory (
RAM-Ram, ram, or RAM as a non-acronymic word:As a non-acronymic word Ram, ram, or RAM may refer to:-Animals:*Bighorn Sheep, A North American mountain sheep species*Sheep, an uncastrated male of which is called a ram...
). This was in contrast to most other CPUs of its time.
The TMS9900 processor's instruction set was
big endianIn computing, endianness is the byte ordering used to represent some kind of data. Typical cases are the order in which integer values are stored as bytes in computer memory and the transmission order over a network or other medium...
and was "clean": all
illegal opcodeAn Illegal Opcode, also called an Undocumented Instruction, is an instruction to a CPU that is not mentioned in any official documentation released by the CPU's designer or manufacturer, which nevertheless has an effect. Illegal opcodes were common on older CPUs designed during the 1970s, such as...
s executed as a
NOPIn computer science NOP or NOOP is an assembly language instruction, sequence of programming language statements, or computer protocol command that effectively does nothing at all.-NOP machine instruction:Some computer instruction sets include an instruction whose explicit purpose is not to...
, and did not produce spurious/unexpected execution results.
Serial I/O was available through address lines.
In typical comparisons with the
Intel 8086The 8086 is a 16-bit microprocessor chip designed by Intel and introduced on the market in 1978, which gave rise to the x86 architecture. Intel 8088, released in 1979, was essentially the same chip, but with an external 8-bit data bus , and is notable as the processor used in the original IBM...
, the TMS9900 had smaller and faster programs. The only disadvantage was the small address space and need for fast RAM.
The TMS9900 had an uncommon, and rarely used, instruction called 'X' (or eXecute). This instruction was used to
executeExecution in computer and software engineering is the process by which a computer or a virtual machine carries out the instructions of a computer program. The instructions in the program trigger sequences of simple actions on the executing machine...
another instruction at an address pointed to by a
registerIn computer architecture, a processor register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage available elsewhere. Most, but not all, modern computers adopt the so-called load-store architecture...
. Unlike more common jump or branch instructions, execution then continued with the instruction following the X instruction.
The TMS9900 was used in the TI-99/4 and
TI-99/4AThe Texas Instruments TI-99/4A was an early home computer, released in June 1981, originally at a price of USD $525. It was an enhanced version of the less-successful—and quite rare—TI-99/4 model, which was released in late 1979 at a price of $1,150...
home computers. Unfortunately, to reduce the production costs, TI chose to use in these systems just 128 16-bit words of the fast kind of RAM that the TMS9900 could access directly. The rest of the memory was 16kB of
8-bitEight-bit CPUs normally use an 8-bit data bus and a 16-bit address bus which means that their address space is limited to 64 KB. This is not a "natural law", however, so there are exceptions....
DRAMDynamic random access memory is a type of random access memory that stores each bit of data in a separate capacitor within an integrated circuit. Since real capacitors leak charge, the information eventually fades unless the capacitor charge is refreshed periodically...
that was accessible only through the video display controller, which crippled the performance of the TMS9900.
Despite the very poor support from Texas Instruments, the TMS9900 had the potential at one point to surpass the
8086The 8086 is a 16-bit microprocessor chip designed by Intel and introduced on the market in 1978, which gave rise to the x86 architecture. Intel 8088, released in 1979, was essentially the same chip, but with an external 8-bit data bus , and is notable as the processor used in the original IBM...
in popularity.
TI later developed the more powerful and more capable TMS99000, which was used as the CPU in the 990/10A minicomputer as a cost reduction. Unfortunately, by the time the 990/10A made it to market, the end of the minicomputer era was already in sight.
External links