Texas Instruments TMS9900
|General purpose||16 × 16-bit in external RAM|
|PC, WP, ST|
Introduced in June 1976, the TMS9900 was one of the first commercially available, single-chip 16-bit microprocessors. The TMS9900 found its most widespread use in the TI-99/4 and TI-99/4A home computers.
The TMS9900 was designed as a single chip version of the TI 990 minicomputer series, much like the Intersil 6100 was a single chip PDP-8 (12 bit), and the Fairchild 9440 and Data General mN601 were both one-chip versions of Data General's Nova. Unlike other 16-bit microprocessors such as the National Semiconductor IMP-16 or DEC LSI-11, some of which predated the TMS9900, the latter was a single-chip, self-contained 16-bit microprocessor.
The TMS9900 has three internal 16-bit registers — Program counter (PC), Status register (ST), and Workspace Pointer register (WP). The WP register points to a base address in external RAM where the processor's 16 general purpose user registers (each 16 bits wide) are kept. This architecture allows for quick context switching; e.g. when a subroutine is entered, only the single workspace register needs to be changed instead of requiring registers to be saved individually.
Addresses refer to bytes with big endian ordering convention. The TMS9900 is a classic 16 bit machine with an address space of 216 bytes (65,536 bytes or 32,768 words).
There is no concept of a stack and no stack pointer register. Instead, Branch instructions exist that save the program counter to a register and change the register context. The 16 hardware and the 16 software interrupt vectors each consist of a pair of PC and WP values, so the register context switch is automatically performed by an interrupt as well.
Instruction set and addressing 
The TMS9900 has 72 instructions which are one, two or three words long and always word-aligned in memory. The instruction set is fairly orthogonal, meaning that with few exceptions, instructions can use all methods of accessing operands (addressing modes).
Addressing modes include Immediate (operand in instruction), Direct or "Symbolic" (operand address in instruction), Register (operand in workspace register), Register Indirect (operand address in workspace register) with or without auto-increment, Indexed (operand address in instruction indexed with workspace register content), and Program Counter Relative.
The most important dual-operand instructions (add, subtract, compare, move etc.) contain 2-bit addressing mode and 4-bit register selector fields for both source and destination operands. In the opcode, "Symbolic" mode is represented as Indexed mode with the register field set to 0, therefore workspace register 0 (WR0) cannot be used in Indexed mode. In less frequently used dual-operand instructions like XOR, the destination operand must be a workspace register (or workspace register pair in the case of multiply and divide instructions).
Flow control is facilitated through a group of one unconditional and twelve conditional Jump instructions. Jump targets are relative to PC with an offset of -128 to +127 word addresses.
For subroutine calls, the Branch and Load Workspace Pointer (BLWP) instruction loads new WP and PC values, then saves the values of WP, PC and ST to the (new) registers 13, 14 and 15 respectively. At the end of the subroutine, the Return Workspace Pointer (RTWP) restores these in reverse order. Using BLWP/RTWP, it is possible to nest subroutine calls despite the absence of a stack, however, the programmer needs to assign the appropriate register workspace explicitly.
The instruction set also contains a Branch and Link (BL) opcode that only saves PC to register 11 without changing WP. In this case, another BL instruction using WR11 as the destination address can serve as the return opcode, but BL-type subroutines cannot be nested.
The TMS9900 has an uncommon, and rarely used, instruction called "X" (eXecute). This instruction is used to execute another instruction at an address pointed to by a register without changing program flow, i.e. execution continues at the address following the X instruction. This instruction can be used for debugging (as a breakpoint instruction) and for creating indexed-opcode tables as used in byte-code interpreters.
The TMS9900 was implemented in an N-channel silicon gate MOS process, which required +5V, -5V and +12V power supplies and a four-phase (non-overlapping) clock with a maximum frequency of 3 MHz (333ns cycle), usually generated from a 48 MHz crystal using a TIM9904 (aka 74LS362) clock generator chip.
The shortest instructions require eight clock cycles or 2.7 μs to complete (assuming 0 external wait cycles), many others run between 10 and 14 cycles (3.3-4.7 μs); the longest-running instruction (DIV) can take up to 124 cycles (41.3 μs).
The chip was packaged in a (then unusual) 64-pin, 0.9" wide DIP. The comparatively large number of pins allowed for the 15-bit (word) address bus and 16-bit data bus to be brought out on dedicated pins without the use of multiplexing (unlike e.g. the Intel 8086 CPU), keeping external memory connections simple. Contrary to the convention used by many other manufacturers, TI labeled the most significant address and data lines "A0" and "D0", respectively. All internal data paths and the ALU are 16 bits wide.
The processor can be paused with the address bus tri-stated for external direct memory access (DMA). Memory accesses are always 16 bits wide, with the CPU automatically performing read-before-write operations for byte-wide accesses.
The hardware interrupt system supports a 4-bit interrupt priority input, which needed to be higher than the priority level stored in the status register (bits 12-15) in order for the interrupt request to be served. In addition, the /LOAD input provides a non-maskable interrupt facility with a dedicated vector.
The TMS9900 CPU also contains a 16-bit shift register ("CRU") designed for interfacing with external shift registers, with dedicated instructions supporting access to fields of 1-16 bit width out of a total of 4096 addressable bits.
Parallel peripherals can be attached in memory-mapped fashion to the regular address and data bus.
The TMS9900 was used in the TI-99/4 and TI-99/4A 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-bit DRAM that was accessible only indirectly through the video display controller, which crippled the performance of the TI-99/4.
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.
|TI990/9||Early multi-chip CPU for minicomputer systems, 1974|
|TI990/10||Multi-chip implementation for minicomputer systems, 1975|
|TI990/12||Multi-chip implementation, faster than 990/10.|
|TMS9900||Single chip implementation, 1976. Used in the TI-99/4(A) computer.|
|TMS9940||Microcontroller with 2KB ROM, 128B RAM, decrementer, CRU bus, 1979|
|TMS9980, TMS9981||8-bit versions of TMS9900|
|TMS9985||A TMS9940 with 8KB ROM, 256B RAM, and a 8-bit external bus, c. 1978 (never released)|
|TMS9989||Improved 9980, used in military hardware.|
|TMS9995||Improved TMS9985-like, no ROM. Used in the TI-99/2 & TI-99/8 prototypes, and the Geneve computer.|
|TMS99000||Improved single-chip 9900, intended to replace 990/10, 1981|
|TMS99105||can emulate additional instructions|
- TMS9900 Microprocessor Data Manual. Texas Instruments Inc. 1976. p. 3.
- TMS9900 Microprocessor Data Manual. Texas Instruments Inc. 1976. p. 3.
- TMS9900 Microprocessor Data Manual. Texas Instruments Inc. 1976. p. 31.
- TMS9900 Microprocessor Data Manual. Texas Instruments Inc. 1976. p. 28.
- TMS9900 Microprocessor Data Manual. Texas Instruments Inc. 1976. p. 6.