Editing Transputer (section)

=== Instruction set ===
To include all this function on one chip, the transputer's core logic was simpler than most CPUs. While some have called it [[reduced instruction set computer]] (RISC) due to its rather sparse nature, and because that was then a desirable marketing [[buzzword]], it was heavily [[microcode]]d, had a limited register set, and complex memory-to-memory instructions, all of which place it firmly in the [[complex instruction set computer|CISC]] camp. Unlike register-heavy [[Load/store architecture|load/store RISC]] CPUs, the transputer had only three data registers, which behaved as a stack. In addition a workspace pointer pointed to a conventional memory stack, easily accessible via the instructions <code>Load Local</code> and <code>Store Local</code>. This allowed for very fast [[context switch]]ing by simply changing the workspace pointer to the memory used by another process (a method used in a number of contemporary designs, such as the [[TMS9900]]). The three register stack contents were not preserved past certain instructions, like Jump, when the transputer could do a context switch.

The transputer instruction set consisted of 8-bit instructions assembled from [[opcode]] and [[operand]] [[nibble]]s. The ''upper'' nibble contained the 16 possible primary instruction codes, making it one of the very few commercialized [[minimal instruction set computer]]s. The ''lower'' nibble contained the one immediate constant operand, commonly used as an offset relative to the workspace (memory stack) pointer. Two [[prefix]] instructions allowed construction of larger constants by prepending their lower nibbles to the operands of following instructions. Further instructions were supported via the instruction code ''Operate'' (<code>Opr</code>), which decoded the constant operand as an extended zero-operand opcode, providing for almost endless and easy instruction set expansion as newer implementations of the transputer were introduced.

The 16 'primary' one-operand instructions were:

{| class="wikitable"
|-
! Mnemonic
! Description
|-
|J
|Jump – add immediate operand to instruction pointer
|-
|LDLP
|Load local pointer – load a workspace-relative pointer onto the top of the register stack
|-
|PFIX
|Prefix – general way to increase lower nibble of following primary instruction
|-
|LDNL
|Load non-local – load a value offset from address at top of stack
|-
|LDC
|Load constant – load constant operand onto the top of the register stack
|-
|LDNLP
|Load non-local pointer – load address, offset from top of stack
|-
|NFIX
|Negative prefix – general way to negate (and possibly increase) lower nibble
|-
|LDL
|Load local – load value offset from workspace
|-
|ADC
|Add constant – add constant operand to top of register stack
|-
|CALL
|Subroutine call – push instruction pointer and jump
|-
|CJ
|Conditional jump – depending on value at top of register stack
|-
|AJW
|Adjust workspace – add operand to workspace pointer
|-
|EQC
|Equals constant – test if top of register stack equals constant operand
|-
|STL
|Store local – store at constant offset from workspace
|-
|STNL
|Store non-local – store at address offset from top of stack
|-
|OPR
|Operate – general way to extend instruction set
|}

All these instructions take a constant, representing an offset or an arithmetic constant. If this constant was less than 16, all these instructions coded to one byte.

The first 16 'secondary' zero-operand instructions (using the OPR primary instruction) were:
{| class="wikitable"
|-
! Mnemonic
! Description
|-
|REV
|Reverse – swap two top items of register stack
|-
|LB
|Load byte
|-
|BSUB
|Byte subscript
|-
|ENDP
|End process
|-
|DIFF
|Difference
|-
|ADD
|Add
|-
|GCALL
|General call – swap top of stack and instruction pointer
|-
|IN
|Input – receive message
|-
|PROD
|Product
|-
|GT
|Greater than – the only comparison instruction
|-
|WSUB
|Word subscript
|-
|OUT
|Output – send message
|-
|SUB
|Subtract
|-
|STARTP
|Start process
|-
|OUTBYTE
|Output byte – send one-byte message
|-
|OUTWORD
|Output word – send one-word message
|}