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The two reserved devices can be used for implementation specific features that do not need to be part of the specs, or other Uxn/Varvara instances.
The two reserved devices can be used for implementation specific features that do not need to be part of the specs, or other Uxn/Varvara instances.


== Uxntal ==


According to the implentation spec, "Tal is the programming language for the Uxn virtual machine." Tal, commonly referred to as '''Uxntal''', is a unique flavor of assembly language. Uxntal source files are assembled from human-readable source files into Uxn-compatible ROM files with the .tal extension.
The language itself works similarly to Forth, another stack-based language.
=== Opcodes ===
Uxn can perform 32 different operations, and each operation has 3 possible modes. The items in the following table include the Program Counter(PC), Memory(M), Devices(D) and Return Stack(rs).
{| class="wikitable"
|-
!colspan="2"|Stack
!colspan="2"|Memory
|-
| '''BRK/LIT''' - Literal
| a b c M[PC+1]
| '''LDZ''' - Load Zeropage
| a b M[c8]
|-
| '''INC''' - Increment
| a b c+1
| '''STZ''' - Save Zeropage
| a {M[c8]=b}
|-
| '''POP''' - Pop
| a b
| '''LDR''' - Load Rel
| a b M[PC+c8]
|-
| '''NIP''' - Nip
| a c
| '''STR''' - Save Rel
| a {M[PC+c8]=b}
|-
| '''SWP''' - Swap
| a c b
| '''LDA''' - Load Abs
| a b M[c16]
|-
| '''ROT''' - Rotate
| c b a
| '''STA''' - Save Abs
| a {M[c16]=b}
|-
| '''DUP''' - Duplicate
| a b c c
| '''DEI''' - Device In
| a b D[c8]
|-
| '''OVR''' - Over
| a b c b
| '''DEO''' - Device Out
| a {D[c8]=b}
|-
!colspan="2"|Logic
!colspan="2"|Arithmetic
|-
| '''EQU''' - Equal
| a b?c
| '''ADD''' - Add
| a b+c
|-
| '''NEQ''' - Not Equal
| a b!c
| '''SUB''' - Subtract
| a b-c
|-
| '''GTH''' - Greater
| a b>c
| '''MUL''' - Multiply
| a b*c
|-
| '''LTH''' - Lesser
| a b<c
| '''DIV''' - Divide
| a b/c<br />
|-
| '''JMP''' - Jump<br />
| a b {PC+=c}
| '''AND''' - And
| a b&c
|-
| '''JCN''' - JumpCond
| a {(b8)PC+=c}
| '''ORA''' - Or
| <nowiki>a b|c</nowiki>
|-
| '''JSR''' - JumpStash
| a b {rs.PC PC+=c}
| '''EOR''' - ExclusiveOr
| a b^c
|-
| '''STH''' - Stash
| a b {rs.c}
| '''SFT''' - Shift
| a b>>c8l<<c8h
|}


The Uxn CPU reads one byte at a time from the main memory starting at 0100. The program counter is a 16-bit word that indicates the address of the byte to read next. Each byte read corresponds to an opcode which  may cause a change in the stack(s) or the normal flow of the program counter; instead of pointing to the next byte in memory, it can be made to point elsewhere, "jumping" from one place in memory to another.  
The Uxn CPU reads one byte at a time from the main memory starting at 0100. The program counter is a 16-bit word that indicates the address of the byte to read next. Each byte read corresponds to an opcode which  may cause a change in the stack(s) or the normal flow of the program counter; instead of pointing to the next byte in memory, it can be made to point elsewhere, "jumping" from one place in memory to another.  
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