Uxntal Opcodes

Uxn has 64kb of memory, 16 devices, 2 stacks of 256 bytes, 5-bits opcodes and 3 modes. The list below show the standard opcodes and their effect on a given stack a b c. PC: Program Counter, |: Return Stack, [M]: Memory, [D+*]: Device Memory, a8: a byte, a16: a short.

LIT a b c [PC]
JCI a b (c8)PC+=[PC]
JMI a b c PC+=[PC]
JSI a b c | PC PC+=[PC]

BRK .        EQU a b==c          LDZ a b [c8]     ADD a b+c
INC a b c+1  NEQ a b!=c          STZ a [c8]=b     SUB a b-c
POP a b      GTH a b>c           LDR a b [PC+c8]  MUL a b*c
NIP a c      LTH a b<c           STR a [PC+c8]=b  DIV a b/c
SWP a c b    JMP a b PC+=c       LDA a b [c16]    AND a b&c
ROT b c a    JCN a (b8)PC+=c     STA a [c16]=b    ORA a b|c
DUP a b c c  JSR a b | PC PC+=c  DEI a b [D+c8]   EOR a b^c
OVR a b c b  STH a b | c         DEO a [D+c8]=b   SFT a b>>c8l<<c8h

Modes

Each opcode has 3 possible modes, which can combined:

• The short mode 2 operates on shorts, instead of bytes.
• The keep mode k operates without consuming items.
• The return mode r operates on the return stack.

By default, operators consume bytes from the working stack, notice how in the following example only the last two bytes #45 and #67 are added, even if there are two shorts on the stack.

#1234 #4567 ADD12 34 ac

The short mode consumes two bytes from the stack. In the case of jump opcodes, the short-mode operation jumps to an absolute address in memory. For the memory accessing opcodes, the short mode operation indicates the size of the data to read and write.

#1234 #4567 ADD2 57 9b

The keep mode does not consume items from the stack, and pushes the result on top. The following example adds the two shorts together, but does not consume them. Under the hood, the keep mode keeps a temporary stack pointer that is decremented on POP.

#1234 #4567 ADD2k 12 34 45 67 57 9b

The return mode makes it possible for any opcode to operate on the return-stack directly. For that reason, there is no dedicated return opcode. For example, the JSR opcode pushes the program's address onto the return stack before jumping, to return to that address, the JMP2r opcode is used, where instead of using the address on the working-stack, it takes its address from the return-stack.

LITr 12 #34 STH ADDr STHr 46

To better understand how the opcode modes are used, here is a 22 bytes long implementation of the function to generate numbers in the Fibonacci sequence. Notice how only a single literal is created to perform the operation.

@fib ( num* -: numfib* )
	#0001 GTH2k ?{ POP2 JMP2r }
	SUB2k fib STH2 INC2
	SUB2 fib STH2r ADD2
	JMP2r

Immediate opcodes

Immediate opcodes are operations which do not take items from the stack, but read values stored immediately after the opcode in the program's memory. Uxntal has 4 immediate opcodes:

• The literal LIT.
• The jump !routine, immediate of JMP.
• The conditional ?routine, immediate of JCN.
• The subroutine routine, immediate of JSR.

The immediate jump opcodes are slightly faster than their standard opcode counterparts, but do not have modes and cannot be used to do pointer arithmetic. The address value of the immediate opcodes are stored in memory as relative shorts, enabling routines making use of these opcodes to be moved around in the program's memory.

@fact ( n* -: res* )
	ORAk ?{ POP2 #0001 JMP2r }
	DUP2 #0001 SUB2 fact MUL2
	JMP2r

Quoting is the act of deferring an operation, for example, by keeping the address to a routine on the stack and using it later, by unquoting it, with the JMP2 or JSR2 opcodes. To learn more about pointer arithmetic, see lambdas.

To learn more about each opcode, see the Opcode Reference.

The Uxntal Opcode Reference

This documentation includes hand gestures, and glyphs, which might serve a dual purpose; both enabling the usage of Uxntal outside of the computer, as well as to help students to familiarize themselves with hexadecimal finger-counting and bitwise operations.

In the a b -- c d notation, "a b" represent the state of the stack before the operation, "c d" represent the state after the operation, with "b" and "d" on top of the stack, respectively.

Break

BRK -- Ends the evalutation of the current vector. This opcode has no modes.

Jump Conditional Instant

JCI cond8 -- Pops a byte from the working stack and if it is not zero, moves the PC to a relative address at a distance equal to the next short in memory, otherwise moves PC+2. This opcode has no modes.

Jump Instant

JMI -- Moves the PC to a relative address at a distance equal to the next short in memory. This opcode has no modes.

Jump Stash Return Instant

JSI -- Pushes PC+2 to the return-stack and moves the PC to a relative address at a distance equal to the next short in memory. This opcode has no modes.

Literal

LIT -- a Pushes the next bytes in memory, and moves the PC forward by the same number of bytes (i.e: 1 byte if short mode is off or 2 bytes if it is on). The LIT opcode always has the keep mode active. Notice how the 0x00 opcode, with the keep bit toggled, is the location of the literal opcodes.

LIT 12          ( 12 )
LIT2 abcd       ( ab cd )

Increment

INC a -- a+1 Increments the value at the top of the stack, by 1.

#01 INC         ( 02 )
#0001 INC2      ( 00 02 )
#0001 INC2k     ( 00 01 00 02 )

Pop

POP a -- Removes the value at the top of the stack.

#1234 POP    ( 12 )
#1234 POP2   ( )
#1234 POP2k  ( 12 34 )

Nip

NIP a b -- b Removes the second value from the stack. This is practical to truncate a short into a byte.

#1234 NIP          ( 34 )
#1234 #5678 NIP2   ( 56 78 )
#1234 #5678 NIP2k  ( 12 34 56 78 56 78 )

Swap

SWP a b -- b a Exchanges the first and second values at the top of the stack.

#1234 SWP          ( 34 12 )
#1234 SWPk         ( 12 34 34 12 )
#1234 #5678 SWP2   ( 56 78 12 34 )
#1234 #5678 SWP2k  ( 12 34 56 78 56 78 12 34 )

Rotate

ROT a b c -- b c a Rotates three values at the top of the stack, to the left, wrapping around.

#1234 #56 ROT            ( 34 56 12 )
#1234 #56 ROTk           ( 12 34 56 34 56 12 )
#1234 #5678 #9abc ROT2   ( 56 78 9a bc 12 34 )
#1234 #5678 #9abc ROT2k  ( 12 34 56 78 9a bc 56 78 9a bc 12 34 )

Duplicate

DUP a -- a a Duplicates the value at the top of the stack.

#1234 DUP   ( 12 34 34 )
#12 DUPk    ( 12 12 12 )
#1234 DUP2  ( 12 34 12 34 )

Over

OVR a b -- a b a Duplicates the second value at the top of the stack.

#1234 OVR          ( 12 34 12 )
#1234 OVRk         ( 12 34 12 34 12 )
#1234 #5678 OVR2   ( 12 34 56 78 12 34 )
#1234 #5678 OVR2k  ( 12 34 56 78 12 34 56 78 12 34 )

Equal

EQU a b -- bool8 Pushes 01 to the stack if the two values at the top of the stack are equal, 00 otherwise.

#1212 EQU          ( 01 )
#1234 EQUk         ( 12 34 00 )
#abcd #ef01 EQU2   ( 00 )
#abcd #abcd EQU2k  ( ab cd ab cd 01 )

Not Equal

NEQ a b -- bool8 Pushes 01 to the stack if the two values at the top of the stack are not equal, 00 otherwise.

#1212 NEQ          ( 00 )
#1234 NEQk         ( 12 34 01 )
#abcd #ef01 NEQ2   ( 01 )
#abcd #abcd NEQ2k  ( ab cd ab cd 00 )

Greater Than

GTH a b -- bool8 Pushes 01 to the stack if the second value at the top of the stack is greater than the value at the top of the stack, 00 otherwise.

#1234 GTH          ( 00 )
#3412 GTHk         ( 34 12 01 )
#3456 #1234 GTH2   ( 01 )
#1234 #3456 GTH2k  ( 12 34 34 56 00 )

Lesser Than

LTH a b -- bool8 Pushes 01 to the stack if the second value at the top of the stack is lesser than the value at the top of the stack, 00 otherwise.

#0101 LTH          ( 00 )
#0100 LTHk         ( 01 00 00 )
#0001 #0000 LTH2   ( 00 )
#0001 #0000 LTH2k  ( 00 01 00 00 00 )

Jump

JMP addr -- Moves the PC by a relative distance equal to the signed byte on the top of the stack, or to an absolute address in short mode.

,&skip-rel JMP BRK &skip-rel #01  ( 01 )

Jump Conditional

JCN cond8 addr -- If the byte preceeding the address is not 00, moves the PC by a signed value equal to the byte on the top of the stack, or to an absolute address in short mode.

#abcd #01 ,&pass JCN SWP &pass POP  ( ab )
#abcd #00 ,&fail JCN SWP &fail POP  ( cd )

Jump Stash Return

JSR addr -- | ret16 Pushes the PC to the return-stack and moves the PC by a signed value equal to the byte on the top of the stack, or to an absolute address in short mode.

,&routine JSR                     ( | PC* )
,&get JSR #01 BRK &get #02 JMP2r  ( 02 01 )

Stash

STH a -- | a Moves the value at the top of the stack to the return stack. Note that with the r-mode, the stacks are exchanged and the value is moved from the return stack to the working stack.

#12 STH       ( | 12 )
LITr 34 STHr  ( 34 )

Load Zero-Page

LDZ addr8 -- value Pushes the value at an address within the first 256 bytes of memory, to the top of the stack.

|00 @cell $2 |0100 .cell LDZ ( 00 )

Store Zero-Page

STZ val addr8 -- Writes a value to an address within the first 256 bytes of memory.

|00 @cell $2 |0100 #abcd .cell STZ2  { ab cd }

Load Relative

LDR addr8 -- value Pushes a value at a relative address in relation to the PC, within a range between -128 and +127 bytes, to the top of the stack.

,cell LDR2 BRK @cell abcd  ( ab cd )

Store Relative

STR val addr8 -- Writes a value to a relative address in relation to the PC, within a range between -128 and +127 bytes.

#1234 ,cell STR2 BRK @cell $2  ( )

Load Absolute

LDA addr16 -- value Pushes the value at a absolute address, to the top of the stack.

;cell LDA BRK @cell abcd ( ab )

Store Absolute

STA val addr16 -- Writes a value to a absolute address.

#abcd ;cell STA BRK @cell $1 ( ab )

Device Input

DEI device8 -- value Pushes a value from the device page, to the top of the stack. The target device might capture the reading to trigger an I/O event.

Device Output

DEO val device8 -- Writes a value to the device page. The target device might capture the writing to trigger an I/O event.

Add

ADD a b -- a+b Pushes the sum of the two values at the top of the stack.

#1a #2e ADD       ( 48 )
#02 #5d ADDk      ( 02 5d 5f )
#0001 #0002 ADD2  ( 00 03 )

Subtract

SUB a b -- a-b Pushes the difference of the first value minus the second, to the top of the stack.

Multiply

MUL a b -- a*b Pushes the product of the first and second values at the top of the stack.

Divide

DIV a b -- a/b Pushes the quotient of the first value over the second, to the top of the stack. A division by zero pushes zero on the stack. The rounding direction is toward zero.

#10 #02 DIV       ( 08 )
#10 #03 DIVk      ( 10 03 05 )
#0010 #0000 DIV2  ( 00 00 )

And

AND a b -- a&b Pushes the result of the bitwise operation AND, to the top of the stack.

Or

ORA a b -- a|b Pushes the result of the bitwise operation OR, to the top of the stack.

Exclusive Or

EOR a b -- a^b Pushes the result of the bitwise operation XOR, to the top of the stack.

Shift

SFT a shift8 -- c Shifts the bits of the second value of the stack to the left or right, depending on the control value at the top of the stack. The high nibble of the control value indicates how many bits to shift left, and the low nibble how many bits to shift right. The rightward shift is done first.

#34 #10 SFT        ( 68 )
#34 #01 SFT        ( 1a )
#34 #33 SFTk       ( 34 33 30 )
#1248 #34 SFTk2    ( 12 48 34 09 20 )

• Rekka Bellum, illustration
• Kira Oakley, contributor
• Ismael Venegas Castello, contributor

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