View source for TMS9900

{{Short description|16-bit microprocessor}}
{{Infobox CPU architecture|name = TMS9900
|designer = [[Texas Instruments]]
|bits = 16-bit
|introduced = 1976
|version =
|design = CISC
|type =
|encoding =
|branching =
|endianness = Big
|extensions =
|open =
|registers = PC, WP, ST
|gpr = 2 internally located in processor (WP, ST) 16 × 16-bit workspace located in external RAM
}}
The '''TMS9900''' was one of the first commercially available, single-chip [[16-bit]] [[microprocessor]]s.{{efn|Only the [[General Instrument CP1600]] is close in release date.}} Introduced in June 1976, it implemented [[Texas Instruments]]' [[TI-990]] [[minicomputer]] architecture in a single-chip format, and was initially used for low-end models of that lineup.
Its 64-pin [[dual in-line package|DIP]] format made it more expensive to implement in smaller machines than the more common 40-pin format, and it saw relatively few design wins outside TI's own use. Among those uses was their [[TI-99/4A|TI-99/4 and TI-99/4A]] home computers, which ultimately sold about 2.8 million units.
Microcomputer-on-chip implementations of the 9900 in 40-pin packages included the TMS9940, TMS9980/81, and TMS9995.
The SBP9900 was a ruggedized version.<ref>{{cite book |title=9900 Family Systems Design and Data Book |date=1978 |publisher=Texas Instruments |isbn=0-89512-026-7 |page=2-3 |url=http://www.bitsavers.org/components/ti/TMS9900/1978_TI_9900_Family_Systems_Design_and_Data_Book_1ed.pdf |access-date=21 November 2024}}</ref>
The last generation was the 99000 series, created to be the CPU of the 990/10A in 1981. The TMS99105 and 110 were sold as catalog parts. <ref name=":0" />
By the mid-1980s the microcomputer field was moving to 16-bit systems like the [[Intel 8086]] and newer 16/32-bit designs like the [[Motorola 68000]].  With no obvious future for the chip, TI's Semiconductor division turned its attention to special-purpose 32-bit processors: the [[Texas Instruments TMS320]], introduced in 1983, and the [[Texas Instruments TMS340]] graphics processor.
The 9900 architecture lived on into the 1990s as the Communications Processor in TI's TMS380 chipset for Token Ring (later Ethernet).<ref>{{cite book |title=TMS380 Adapter Chipset User's Guide |date=July 1986 |publisher=Texas Instruments |pages=A-1-A-22 |url=http://www.bitsavers.org/components/ti/TMS380/ |access-date=21 November 2024}}</ref>
==History==
[[Image:KL TI TMS9900 Black Background.jpg|right|thumb|TMS9900JL in ceramic package with gold-plated [[Lead (electronics)|pin]]s]]
The TMS9900 was designed as a single chip version of the [[TI-990|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 [[Data General Nova|Nova]]. Unlike multi-chip 16-bit microprocessors such as the [[National Semiconductor]] [[IMP-16]] or DEC [[LSI-11]], some of which predated the TMS9900, the 9900 was a single-chip, self-contained 16-bit microprocessor.
The minicomputer roots of the TMS9900 give rise to a number of architectural features that are not commonly found on designs that started from a blank sheet. Notable among these was the TMS9900's use of [[processor register]]s that are mapped into [[main memory]]. This allows for fast [[context switching]], which can be accomplished by changing a single register, the Workspace Pointer, to point to the first entry in a list of register values. More traditional designs would require the entire set of internal registers to be stored out to memory or the stack.
The downside to this approach is that accessing these registers is more time-consuming. In a minicomputer implementation with fast memory, the effect is relatively small and the upside in a real-time or multi-tasking environment is significant as context switches are common. In other roles, like single-user microcomputers, this tradeoff may not be worthwhile. The 40-pin implementations of the 9900 included 128 or 256 bytes of fast onboard RAM for registers.
TI used the same architecture across different divisions for corporate synergy: "one company, one computer architecture". In the late 1970s [[Walden C. Rhines]] gave a presentation of the  TMS99110, then code-named “Alpha”,<ref>{{Cite web |last=Guttag |first=Karl |date=1980-04-14 |title=99000 "Alpha" Performance Comparisons |url=https://hansotten.file-hunter.com/uploads/files/99000%20(Alpha)%20Misc%20Documents.pdf |access-date=2022-10-26}}</ref> to an IBM group developing a personal computer. "We wouldn't know until 1981 just what we had lost" because IBM chose the [[Intel 8088]] for the [[IBM PC]], he recalled.  One factor was the  lack of a roadmap for accessing more than 64K of logical memory.  The 9900 family could expand its address space  to 16MiB only by page-mapping;  the  9900 could address 256K through segments.
After dropping out of the personal computer market with products such as [[TI-99/4A]], the company microprocessor division eventually switched focus to the [[Texas Instruments TMS320|TMS320]] special-purpose processor series.<ref name="rhines20170622">{{Cite magazine |last=Rhines |first=Walden C. |author-link=Walden C. Rhines |date=2017-06-22 |title=The Texas Instruments 99/4: World's First 16-Bit Home Computer |url=http://spectrum.ieee.org/geek-life/history/the-texas-instruments-994-worlds-first-16bit-computer |archive-url=https://web.archive.org/web/20170622212225/http://spectrum.ieee.org/geek-life/history/the-texas-instruments-994-worlds-first-16bit-computer |url-status=dead |archive-date=June 22, 2017 |magazine=IEEE Spectrum |language=en |access-date=2017-07-08}}</ref>
==Architecture==
{| class="infobox" style="font-size:88%;width:31em;"
|-
|+ TMS9900 registers
|-
|
{| style="font-size:88%;"
|-
| style="width:10px; text-align:center;"| <sup>0</sup><sub>0</sub>
| style="width:10px; text-align:center;"| <sup>0</sup><sub>1</sub>
| style="width:10px; text-align:center;"| <sup>0</sup><sub>2</sub>
| style="width:10px; text-align:center;"| <sup>0</sup><sub>3</sub>
| style="width:10px; text-align:center;"| <sup>0</sup><sub>4</sub>
| style="width:10px; text-align:center;"| <sup>0</sup><sub>5</sub>
| style="width:10px; text-align:center;"| <sup>0</sup><sub>6</sub>
| style="width:10px; text-align:center;"| <sup>0</sup><sub>7</sub>
| style="width:10px; text-align:center;"| <sup>0</sup><sub>8</sub>
| style="width:10px; text-align:center;"| <sup>0</sup><sub>9</sub>
| style="width:10px; text-align:center;"| <sup>1</sup><sub>0</sub>
| style="width:10px; text-align:center;"| <sup>1</sup><sub>1</sub>
| style="width:10px; text-align:center;"| <sup>1</sup><sub>2</sub>
| style="width:10px; text-align:center;"| <sup>1</sup><sub>3</sub>
| style="width:10px; text-align:center;"| <sup>1</sup><sub>4</sub>
| style="width:10px; text-align:center;"| <sup>1</sup><sub>5</sub>
| style="width:auto;" | ''(bit position)''
|- style="background:silver;color:black"
| style="text-align:center;" colspan="15"| PC
| style="text-align:center;" colspan="1"| 0
| style="width:auto; background:white; color:black;"| Program Counter
|- style="background:silver;color:black"
| style="text-align:center;" colspan="15"| WP
| style="text-align:center;" colspan="1"| 0
| style="width:auto; background:white; color:black;"| Workspace Pointer
|- style="background:silver;color:black"
| style="text-align:center;"| L>
| style="text-align:center;"| A>
| style="text-align:center;"| [[Zero flag|=]]
| style="text-align:center;"| [[Carry flag|C]]
| style="text-align:center;"| [[Overflow flag|O]]
| style="text-align:center;"| [[Parity flag|P]]
| style="text-align:center;"| X
| style="text-align:center;" colspan="5"| 0
| style="text-align:center;" colspan="4"| Int Mask
| style="background:white; color:black" | Status Register
|- style="background:silver;color:black"
|}
|}
The TMS9900 has three internal 16-bit [[processor register|register]]s — [[Program counter]] (PC), [[Status register]] (ST), and Workspace Pointer register (WP).<ref name="Microprocessor Data Manual p.3">{{cite book|title = TMS9900 Microprocessor Data Manual|publisher = Texas Instruments Inc. | year = 1976| page = 3| url = http://www.bitsavers.org/components/ti/TMS9900/TMS_9900_Microprocessor_Data_Manual_May76.pdf | ref={{sfnref|Texas Instruments|1976}} }}</ref>  The WP register [[pointer (computer programming)|points]] to a base address in external [[Random Access Memory|RAM]] where the processor's 16 general purpose user registers (each 16&nbsp;bits wide) are kept.  This architecture allows for quick [[context switch]]ing; e.g. when a [[subroutine]] is entered, only the single workspace register needs to be changed instead of requiring registers to be saved individually. Bits are numbered unconventionally with the most significant bit being 0.
Addresses refer to bytes with [[endianness|big endian]] ordering convention.  The TMS9900 is a classic 16 bit machine with an address space of 2<sup>16</sup> bytes (65,536 bytes or 32,768 words).
There is no dedicated stack pointer register.  Instead, branch instructions exist that save the program counter to a register (Branch and Link), or change the register context (Branch and Link Workspace, or XOP).  The 16 hardware and 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.  Stacks can be implemented atop either of these mechanisms.<ref>{{cite web |title=DX10 Operating System Release 3.5 System Design Document |url=http://bitsavers.informatik.uni-stuttgart.de/pdf/ti/990/dx10/0939153-9701D_3.5_SystemDesignDocument_Sep82.pdf |publisher=Texas Instruments, Inc. 1982.|page=8.1}}</ref>
==Instruction set and addressing==
The TMS9900 has 69 instructions which are one, two or three words long and always word-aligned in memory.  The instruction set is fairly [[orthogonal instruction set|orthogonal]], meaning that with few exceptions, instructions can use all methods of accessing [[operand]]s (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, a branch instruction (B) using WR11 as the destination address can serve as the return opcode, but BL-type subroutines cannot be nested without the programmer taking actions to save the return address.
The TMS9900 supports an [[execute instruction]] "X" (eXecute). This instruction [[Execution (computers)|executes]] the instruction in a register.  It can be used for debugging (as a [[breakpoint]] instruction), for creating indexed-opcode tables as used in byte-code interpreters and can also be used to perform a time critical I/O instruction during an interrupt.  An example of its utility is shown in the code below where an interrupt being serviced in a very encapsulated manner that would otherwise require many more instructions.
<pre>
;***********************************
;
;	THIS INTERRUPT SIMULATES DMA CONTROL
;	ORGANISED AS FOLLOWS:
;
; R9 HOLDS CURRENT COMMAND, E.G.
;   IOREAD(34B8H): STCR	*R8+,BYTEWIDE	;BYTE WIDE FDC DATA READ
;   IOWRITE(30B8H):LDCR	*R8+,BYTEWIDE	;BYTE WIDE FDC DATA WRITE
; R8 HOLDS THE CURRENT DMA ADDRESS.
; R12 HOLDS THE CURRENT IO PORT - DATREG
;************************************
INTDRQ	X	R9	;CAN BE EITHER READ or WRITE
RTWP
</pre>
this common piece of code during the interrupt that could be used by both I/O read and write commands.  Similar methods could be employed in any debugging methods wanting to be used.
The TMS9900 also supports the eXtended OPeration (XOP) instruction. XOP is given a number in the range 0-15 as well as a source address. When invoked, the instruction will perform a context switch through one of sixteen vectors at predefined locations in memory.  The XOP instruction also places the effective address of the source operand in register 11 of the new workspace.  The context saving feature of the XOP instruction can also be used as to implement inline debugging.<ref>{{cite web | url=https://github.com/AlexanderAdelAU/DEBUGGING_TMS9900_USING_XOP | title=Using TMS 9000 XOP Instruction to implement a Debugging Function| access-date=2024-01-13}}</ref>
XOP is less flexible than a BLWP, as the transfer vectors have to be at fixed locations, but allows one source operand to be directly addressed rather than passed in a register or otherwise.
XOP can be used to implement a system call facility. In TI's DX10 operating system, XOP 15 invokes a system call. A programmer might define an assembler macro, for example SVC, which invokes XOP 15. Another use of XOP was to implement instructions in software which might be handled by dedicated hardware in future versions of the 990 minicomputer series.  An example of such actions can be shown in the code below where a CALL function is implemented using and XOP 6 Instruction.  The beauty of this implementation of a CALL function using and XOP, is that it is straightforward to add checks to determine if the Stack has overflowed; for example C	R10,@2*R9(R13), where R9 points to the address of the stack limit.
<pre>
;
;************************************************
;  CALL SUBROUTINE
;  DEFINE XOP:  DXOP  CALL,6
;  CALLING METHOD:   CALL @SUBROUTINE_ADDRESS
;  R10 <=> STACK POINTER
;*************************************************
;
ED32   C2AD 0014      XOP6: MOV  @2*R10(R13),R10   ;GET STACK POINTER
ED36   064A                 DECT R10               ;DECREMENT STACK POINTER
ED38   C68E          	      MOV  R14,*R10          ;PUSH RETURN PC ONTO STACK
ED3A   C38B                 MOV  R11,R14           ;MOVE EA INTO R14 (PC) FOR CALL
ED3C   CB4A 0014     	      MOV  R10,@2*R10(R13)   ;UPDATE STACK POINTER
ED40   0380                 RTWP                   ;WE ARE NOW USING THE ORIGINAL WP
</pre>
In typical comparisons with the [[Intel 8086]], the TMS9900 had smaller programs.{{citation needed|date=July 2012}} Some{{citation needed|date=January 2018}} disadvantages were the small address space and need for fast RAM.
==Implementation==
[[Image:TMS9900 Pinout.svg|right|thumb|215px|TMS9900 pin configuration (64-Pin [[Dual in-line package|DIP]])]]
The TMS9900 was implemented in an N-channel silicon gate MOS process,<ref name="Microprocessor Data Manual p.3" /> which required +5&nbsp;V, −5&nbsp;V and +12&nbsp;V power supplies and a four-phase (non-overlapping) clock with a maximum frequency of 3&nbsp;MHz (333 ns cycle),{{sfn|Texas Instruments|1976|page = 31}} usually generated from a 48&nbsp;MHz crystal using a TIM9904 (aka 74LS362) clock generator chip.
The shortest instructions require eight clock cycles or 2.7&nbsp;μs to complete (assuming 0 external wait cycles), many others run between 10 and 14 cycles (3.3...4.7&nbsp;μs); the longest-running instruction (DIV) can take up to 124 cycles (41.3&nbsp;μs).{{sfn|Texas Instruments|1976|page = 28}}
The chip was packaged in a (then unusual) 64-pin, 0.9" wide [[dual in-line package|DIP]].  The comparatively large number of pins allowed for the 15-bit (word) [[address bus]] and 16-bit [[Bus (computing)|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 [[Arithmetic logic unit|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.{{sfn|Texas Instruments|1976|page = 6}}
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 I/O|memory-mapped]] fashion to the regular address and data bus.
==Applications==
[[File:Tomy-Tutor-wControllers.jpg|link=File:Tomy-Tutor-wControllers.jpg|thumb|The [[Tomy Tutor]], a 1983 home computer based on the TMS9995 processor]]
The TMS9900 was used in the [[TI-99/4A|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]] [[Dynamic random access memory|DRAM]] that was accessible only indirectly through the video display controller, which crippled the performance of the TI-99/4.
TI developed the TM990 series of computer modules, including CPU, memory, I/O, which when plugged into a card frame could form a 16-bit minicomputer. These were typically used for process control.<ref>{{cite web | url=http://www.stuartconner.me.uk/tm990/tm990.htm | title=Stuart's TM 990 Series 16-Bit Microcomputer Modules| access-date=2014-03-25}}</ref> A microprocessor trainer was released in the form of the TM990/189.<ref>{{cite web | url=http://www.rskey.org/CMS/index.php/exhibit-hall/118-texas-instruments-tm-990189 | title=Programmable Calculators - Texas Instruments TM990/189| access-date=2014-03-25}}</ref>
In the late 1970's [[John_Walker_(programmer) | John Walker]] and Dan Drake developed [[S-100_bus | S100-bus cards]] based on the TMS9900 and a full software stack to go with it.<ref>{{cite web | url=https://www.fourmilab.ch/documents/marinchip/ | title=Marinchip Systems history | access-date=2024-08-25}}</ref><ref>{{cite web | url=https://gitlab.com/marinchip | title=Marinchip software | access-date=2024-08-25}}</ref> They later went on to be co-founders of [[Autodesk]], in part based on software first developed for these TMS9900 based systems.<ref>{{cite web | url=https://www.shapr3d.com/history-of-cad/autodesk-and-autocad | title=Autocad history | access-date=2024-08-25}}</ref>
==TMS9900 Family Product Development==
The second generation of the TMS9900 family of microprocessors was the TMS9995 which provided "functional performance at speeds 3 times faster than any previous 9900 family processor",<ref>{{cite book|title = TMS9995 marketing brochure|publisher = Texas Instruments Inc. | year = 1981| page = 2| url = http://www.stuartconner.me.uk/tms9995_eval_module/downloads/tms9995_evaluation_module_brochure.pdf }}</ref> largely due to the inclusion of instruction prefetch technology. In the home computer arena, the TMS9995 only found use in the [[Tomy Tutor]], an esoteric TI99-4/A upgrade called the [[Geneve 9640]], and a project printed in ''Electronics Today'': the Powertran Cortex.<ref>{{cite web | url=http://www.powertrancortex.com | title=Powertran Cortex.com | access-date=2014-03-27}}</ref> It was planned to be used in the TI-99/2 & TI-99/8 computer systems, but neither advanced past the prototype stage.
[[Image:TI-TMS9900NL.jpg|right|thumb|TMS9900NL in plastic DIP package]]
TI later developed the more powerful TMS99000 family of microprocessors, which was used as the CPU in the [[TI-990#TI-990 models|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.
The TMS99000 family includes two microprocessors,<ref name=":0">{{cite book|title = TMS99105A and TMS99110A 16-Bit Microprocessors Preliminary Data Manual|publisher = Texas Instruments Inc. | year = 1982| page = 1| url = http://bitsavers.org/components/ti/TMS9900/TMS99105A_and_TMS99110A_16-Bit_Microprocessors_Preliminary_Data_Manual_Nov82.pdf }}</ref> the TMS99105A and the TMS99110A, which are identical except for the contents of on-chip macrostore ROM memory (macrostore memory contains added functions or instructions through emulation routines written in standard machine code). The on-chip ROM Macrostore in the TMS99110A microprocessor contains floating point instructions which are available as part of the machine language instruction set, while the baseline TMS99105A does not. Both chips can implement  Macrostore instructions in an external ROM. A third member of the TMS99000 family, the TMS99120, was announced<ref>{{cite book|title = Microsystems Designers Handbook, 2nd Edition|publisher = Texas Instruments Limited | year = 1981 | pages = 3–25 onwards }}</ref> but may never have been commercially produced. The on-chip ROM Macrostore in the TMS99120 was to contain run-time support routines for the PASCAL high-level language.
The instruction set for the TMS99000 family extends the 9900 instruction set while keeping compatibility. The additional instructions includes those for signed multiply and divide (first appearing in the TMS9995), long-word shift, add, and subtract; load status register, load workspace pointer, stack operations, multiprocessor support, bit manipulation. Members of the family can access 256KB of memory through code/data segmentation, and may use the TIM99610 memory mapper to address up to 16MiB. The architecture contains many other advances over the TMS9900 and TMS9995.<ref name=":0" />
==Variants==
{| class="wikitable"
! Model
! Description
|-
| 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<br />TMS9981  ||   8-bit databus versions of TMS9900
|-
| TMS9985  || TMS9940 with 8KB ROM, 256B RAM, and an 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, the [[Tomy Tutor]] and the Geneve computer [[Geneve 9640]]
|-
| TMS99105 || Baseline member of the TMS99000 microprocessor family
|-
| TMS99110 || TMS99000 family microprocessor with floating point instructions pre-programmed into the on-chip Macrostore ROM memory
|-
| TMS99120 || TMS99000 family microprocessor with run-time support routines for the PASCAL high-level language pre-programmed into<br />the on-chip Macrostore ROM memory (microprocessor was announced but may never have been commercially produced)
|-
|}
<gallery caption="Die photos" mode="packed" heights="200px">
Image:TI_TMS9900_die.JPG|TMS9900
Image:TI_TMS9981_die.jpg|TMS9981
Image:Rochester_Electronics_TMS9995_die.JPG|TMS9995
Image:TI_TMS99105A_die.JPG|TMS99105A
</gallery>
==Notes==
{{notelist}}
==References==
{{reflist}}
==External links==
*[http://www.bitsavers.org/components/ti/TMS9900 TMS9900 manuals and references] from Bitsavers
{{Authority control}}
[[Category:TI-99/4A]]
[[Category:Texas Instruments microprocessors]]
[[Category:16-bit microprocessors]]