Editing Varvara (section)

== 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. 


[[Category: Fantasy computers]]