uxntal manpage
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.\" Manpage reference for uxntal.
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.\" Contact d_m@plastic-idolatry.com to correct errors or typos.
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.TH uxntal 1 "01 Aug 2024" "1.0" "Uxntal Reference Guide"
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.SH NAME
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uxntal \- assembly langauge for Varvara virtual machine
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.SH DESCRIPTION
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Uxntal is an 8-bit instruction set for programming the Varvara virtual machine.
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It uses the lower 5-bits to specify an opcode, and the upper 3-bits to specify
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optional modes.
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ROMs consist of a 16-bit address space of bytes. Any byte can be interpreted as either data or an instruction. A 2-byte instruction pointer determines the address of the next instruction to decode and run.
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Instructions manipulate data using two stacks: a working stack (\fBwst\fP) and a return stack (\fBrst\fP). Each stack consists of 256 bytes, and in the case of overflow or underflow the stack pointer will wrap (the stacks are circular).
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.SH INSTRUCTION LAYOUT
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0x01 -
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0x02 \\
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0x04 +- \fIopcode\fP
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0x08 /
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0x10 -
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0x20 ---- 2: \fIshort\fP mode
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0x40 ---- r: \fIreturn\fP mode
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0x80 ---- k: \fIkeep\fP mode
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.SH OPCODE LAYOUT
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There are 32 base values for opcodes:
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0x00 \fB***\fP 0x08 \fBEQU\fP 0x10 \fBLDZ\fP 0x18 \fBADD\fP
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0x01 \fBINC\fP 0x09 \fBNEQ\fP 0x11 \fBSTZ\fP 0x19 \fBSUB\fP
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0x02 \fBPOP\fP 0x0a \fBGTH\fP 0x12 \fBLDR\fP 0x1a \fBMUL\fP
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0x03 \fBNIP\fP 0x0b \fBLTH\fP 0x13 \fBSTR\fP 0x1b \fBDIV\fP
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0x04 \fBSWP\fP 0x0c \fBJMP\fP 0x14 \fBLDA\fP 0x1c \fBAND\fP
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0x05 \fBROT\fP 0x0d \fBJCN\fP 0x15 \fBSTA\fP 0x1d \fBORA\fP
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0x06 \fBDUP\fP 0x0e \fBJSR\fP 0x16 \fBDEI\fP 0x1e \fBEOR\fP
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0x07 \fBOVR\fP 0x0f \fBSTH\fP 0x17 \fBDEO\fP 0x1f \fBSFT\fP
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The "complete" opcode's value can be derived by combining the base value with its flags.
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For example, \fBADD2k\fP is \fB(0x18 | 0x20 | 0x80)\fP = \fB0xb8\fP.
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Unlike other opcodes, \fB0x00\fP is contextual. Its meaning depends on the \fImode\fP bits provided:
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0x00 \fBBRK\fP 0x80 \fBLIT\fP
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0x20 \fBJCI\fP 0xa0 \fBLIT2\fP
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0x40 \fBJMI\fP 0xc0 \fBLITr\fP
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0x60 \fBJSI\fP 0xe0 \fBLIT2r\fP
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.SH REGULAR INSTRUCTIONS
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Regular instructions have a single stack effect which is modified in a predictable way by any additional modes.
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For example the generic effect for \fBADD\fP is ( x y -- x+y ). The eight combinations of modes have the following effects:
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\fBADD\fP ( x^ y^ -- x+y^ ) sum two bytes using \fBwst\fP
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\fBADDr\fP ( [x^ y^] -- [x+y^] ) sum two bytes using \fBrst\fP
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\fBADD2\fP ( x* y* -- x+y* ) sum two shorts using \fBwst\fP
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\fBADD2r\fP ( [x* y*] -- [x+y*] ) sum two shorts using \fBrst\fP
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\fBADDk\fP ( x^ y^ -- x^ y^ x+y^ ) sum two bytes using \fBwst\fP, retain arguments
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\fBADDkr\fP ( [x^ y^] -- [x^ y^ x+y^] ) sum two bytes using \fBrst\fP, retain arguments
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\fBADD2k\fP ( x* y* -- x* y* x+y* ) sum two shorts using \fBwst\fP, retain arguments
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\fBADD2kr\fP ( [x* y*] -- [x* y* x+y*] ) sum two shorts using \fBrst\fP, retain arguments
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Thus for regular instructions writing a "generic" effect (leaving sigils off values whose size depends on \fIshort\fP mode) is sufficient to describe its behavior across all eight variations. Note that some instructions always read values of a fixed size. (For example the boolean condition read by \fBJCN\fP is always one byte, no matter what modes are used.)
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.SS INC
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( x -- x+1 )
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.SS POP
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( x -- )
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.SS NIP
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( x y -- y )
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.SS SWP
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( x y -- y x )
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.SS ROT
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( x y z -- y z x )
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.SS DUP
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( x -- x x )
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.SS OVR
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( x y -- x y x )
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.SH SPECIAL INSTRUCTIONS
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.SS BRK
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The break instruction is used to end a vector call and return control to the virtual machine.
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.SS JCI, JMI, and JSI
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The "immediate jump" instructions are produced by the assembler. They interpret the next 2 bytes of the ROM as a relative address (\fIaddr\fP) and have the following effects:
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\fBJMI\fP ( -- ) jump to \fIaddr\fP unconditionally
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\fBJCI\fP ( bool^ -- ) jump to \fIaddr\fP if \fIbool\fP is non-zero
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\fBJSI\fP ( -- [cur] ) jump to \fIaddr\fP saving the current address (\fIcur\fP) on the return stack
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(The instruction pointer will be moved forward 2 bytes, past the relative address.)
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.SS LIT, LIT2, LITr, and LIT2r
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The "literal" instructions are used to push new data onto the stacks. They interpret the next 1-2 bytes of the ROM (\fIwx\fP, \fIwxyz\fP) as data and push it onto the corresponding stack:
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\fBLIT\fP ( -- wx^ ) push literal byte \fIwx\fP onto the \fBwst\fP
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\fBLITr\fP ( -- [wx^] ) push literal byte \fIwx\fP onto the \fBrst\fP
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\fBLIT2\fP ( -- wxyz* ) push literal short \fIwxyz\fP (2 bytes) onto the \fBwst\fP
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\fBLIT2r\fP ( -- [wxyz*] ) push literal short \fIwxyz\fP (2 bytes) onto the \fBrst\fP
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(The instruction pointer will be moved forward 1-2 bytes, past the literal data.)
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