337 lines
13 KiB
Groff
337 lines
13 KiB
Groff
.\" Manpage reference for uxntal.
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.\" by Eiríkr Åsheim
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.\" Contact d_m@plastic-idolatry.com to correct errors or typos.
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.TH uxntal 7 "05 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 program counter (\fIpc\fP) 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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There are also 256 bytes of device memory, which are used to interact with the virtual machine and its devices.
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Instructions deal with unsigned 8-bit values (\fIbytes\fP) and unsigned 16-bit values (\fIshorts\fP). There are no other built-in data types.
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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 mode\fP
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0x40 ---- r: \fIreturn mode\fP
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0x80 ---- k: \fIkeep mode\fP
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.SH OPCODE LAYOUT
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There are 32 base values for opcodes:
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0x00 \fBBRK*\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(ADD | 2 | k)\fP = \fB(0x18 | 0x20 | 0x80)\fP = \fB0xb8\fP.
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Unlike other opcodes, \fB0x00\fP (\fBBRK*\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 STACK EFFECTS
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.BR
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.SS NOTATION
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Given a stack effect \fB( a^ b^ c^ -- c^ a^ b^ )\fP here is what each symbol means:
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\fB(\fP and \fB)\fP are comment delimiters
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\fBa^\fP, \fBb^\fP, and \fBc^\fP are values on the stack
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\fB^\fP indicates that each value is a \fIbyte\fP (\fB*\fP would indicate \fIshort\fP)
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\fB--\fP separates the "before" and "after" of the stack effect
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The effect here is to move the top byte of the stack below the next two bytes, which could be achieved with \fBROT ROT\fP.
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By default stack effects describe the effect on \fBwst\fP. When \fBrst\fP is involved we use \fB[]\fP to differentiate the stacks. For example \fB( a* [b*] -- a+1* [b+1*] )\fP will increment the top short of both \fBwst\fP and \fBrst\fP.
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.SS EFFECTS AND MODES
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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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.nf
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\fBADD\fP ( x^ y^ -- x+y^ ) sum bytes from \fBwst\fP
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\fBADDr\fP ( [x^ y^] -- [x+y^] ) sum bytes from \fBrst\fP
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\fBADD2\fP ( x* y* -- x+y* ) sum shorts from \fBwst\fP
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\fBADD2r\fP ( [x* y*] -- [x+y*] ) sum shorts from \fBrst\fP
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\fBADDk\fP ( x^ y^ -- x^ y^ x+y^ ) sum and keep bytes from \fBwst\fP
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\fBADDkr\fP ( [x^ y^] -- [x^ y^ x+y^] ) sum and keep bytes from \fBrst\fP
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\fBADD2k\fP ( x* y* -- x* y* x+y* ) sum and keep shorts from \fBwst\fP
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\fBADD2kr\fP ( [x* y*] -- [x* y* x+y*] ) sum and keep shorts from \fBrst\fP
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.fi
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Thus for regular instructions writing a "generic" effect (leaving sigils off values whose size depends on \fIshort mode\fP) 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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In \fIreturn mode\fP the stacks are reversed. Effects on \fBwst\fP will instead affect \fBrst\fP, and effects on \fBrst\fP will instead affect \fBwst\fP. For example, \fBSTH\fP reads a byte from \fBwst\fP and writes it to \fBrst\fP, but \fBSTHr\fP reads a byte from \fBrst\fP and writes it to \fBwst\fP.
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In \fIkeep mode\fP all the values on the left-hand side of the stack effect will also appear on the right-hand side before the outputs. For example, \fBSWP\fP is \fB(x y -- y x)\fP but \fBSWPk\fP is \fB(x y -- x y y x)\fP.
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.SS TERMINOLOGY
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We consider the top of the stack to be the first value of the stack, and count back from there. For example, given the stack effect \fB( a b c -- )\fP we would say that \fBc\fP is the top of the stack, \fBb\fP is the second value (second from the top), and \fBa\fP is the third value (third from the top).
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.SH REGULAR INSTRUCTIONS
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.BR
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.SS INC ( x -- x+1 )
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Increment the top value of the stack by 1.
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Overflow will be truncated, so \fB#ff INC\fP will evaluate to \fB0x00\fP.
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.SS POP ( x -- )
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Remove the top value of the stack.
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\fBPOPk\fP is guaranteed to have no effect (it will not change the stack).
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.SS NIP ( x y -- y )
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Remove the second value of the stack.
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\fBNIPk\fP is guaranteed to have no effect (it will not change the stack).
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.SS SWP ( x y -- y x )
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Swap the top two values of the stack.
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.SS ROT ( x y z -- y z x )
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Rotate the top three values of the stack. The lowest becomes the top and the others are each shifted down one place.
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.SS DUP ( x -- x x )
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Place a copy of the top value of the stack on top of the stack.
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.SS OVR ( x y -- x y x )
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Place a copy of the second value of the stack on top of the stack.
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.SS EQU ( x y -- x==y^ )
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Test whether the top two values of the stack are equal.
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Result is guaranteed to be boolean (\fB0x00\fP or \fB0x01\fP).
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.SS NEQ ( x y -- x!=y^ )
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Test whether the top two values of the stack are not equal.
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Result is guaranteed to be boolean (\fB0x00\fP or \fB0x01\fP).
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.SS GTH ( x y -- x>y^ )
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Test whether the second value of the stack is greater than the top.
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Result is guaranteed to be boolean (\fB0x00\fP or \fB0x01\fP).
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.SS LTH ( x y -- x<y^ )
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Test whether the second value of the stack is less than the top.
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Result is guaranteed to be boolean (\fB0x00\fP or \fB0x01\fP).
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.SS JMP ( x -- ; pc <- x )
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Jump to a location.
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The program counter (\fIpc\fP) is unconditionally updated. When \fIx\fP is a byte, it is treated as relative (\fBpc += x\fP) and when \fIx\fP is a short it is treated as absolute (\fBpc = x\fP).
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It is common to \fBJMP\fP with boolean bytes (0-1) to handle simple conditionals. For example:
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@max ( x^ y^ -- max^ ) GTHk JMP SWP POP JMP2r
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.SS JCN ( x bool^ -- ; pc <- x if bool )
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Jump to a location when a condition is true.
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The program counter (\fIpc\fP) is updated when \fIbool\fP is non-zero. When \fIx\fP is a byte, it is treated as relative (\fBpc += x\fP) and when \fIx\fP is a short it is treated as absolute (\fBpc = x\fP).
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.SS JSR ( x -- [pc+1*] )
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Jump to a location, saving a reference to return to.
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Stores the next address to execute before unconditionally updating the program counter (\fIpc\fP). This instruction is usually used to invoke subroutines, which use the \fBJMP2r\fP to return. When \fIx\fP is a byte, it is treated as relative (\fBpc += x\fP) and when \fIx\fP is a short it is treated as absolute (\fBpc = x\fP).
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The saved address will always be a short regardless of \fIshort mode\fP.
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.SS STH ( x -- [x] )
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Move the top value of the stack to the return stack.
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.SS LDZ ( zp^ -- x )
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Load data from a zero-page address (\fB0x00 - 0xff\fP).
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.SS STZ ( x zp^ -- )
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Store data at a zero-page address (\fB0x00 - 0xff\fP).
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.SS LDR ( rel^ -- x )
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Load data from a relative address (\fBpc + x\fP).
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Note that unlike \fBLDZk\fP and \fBLDAk\fP the \fBLDRk\fP instruction is not very useful, since a relative address is usually only meaningful when run from a particular address (i.e. for a particular \fIpc\fP value).
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.SS STR ( x rel^ -- )
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Store data at a relative address (\fBpc + x\fP).
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Note that unlike \fBSTZk\fP and \fBSTAk\fP the \fBSTRk\fP instruction is not very useful, since a relative address is usually only meaningful when run from a particular address (i.e. for a particular \fIpc\fP value).
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.SS LDA ( abs* -- x )
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Load data from an absolute address (\fB0x0000 - 0xffff\fP).
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.SS STA ( x abs* -- )
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Store data at an absolute address (\fB0x0000 - 0xffff\fP).
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.SS DEI ( dev^ -- x )
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Read data from a device port (\fB0x00 - 0xff\fP).
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Reading from some ports may have an effect on the underlying VM; in other cases it will simply read values from device memory. See Varvara device documentation for more details.
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.SS DEO ( x dev^ -- )
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Write data to a device port (\fB0x00 - 0xff\fP).
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Writing to some ports may have an effect on the underlying VM; in other cases it will simply write values to device memory. See Varvara device documentation for more details.
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.SS ADD ( x y -- x+y )
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Add the top two values of the stack.
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Overflow will be truncated, so \fB#ff #03 ADD\fP will evaluate to \fB0x02\fP.
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.SS SUB ( x y -- x-y )
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Subtract the top of the stack from the second value of the stack.
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Underflow will be truncated, so \fB#01 #03 SUB\fP will evaluate to \fB0xfe\fP.
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.SS MUL ( x y -- xy )
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Multiply the top two values of the stack.
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Overflow will be truncated, so \fB#11 #11 MUL\fP will evaluate to \fB0x21\fP.
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.SS DIV ( x y -- x/y )
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Divide the second value of the stack by the top of the stack.
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\fBDIV\fP implements \fIEuclidean division\fP, which is also known as \fIinteger division\fP. It returns whole numbers, so \fB#08 #09 DIV\fP evaluates to \fB0x00\fP.
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Division by zero will return zero (instead of signaling an error).
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Unlike \fBADD\fP, \fBSUB\fP, and \fBMUL\fP, \fBDIV\fP does not behave correctly for numbers which should be treated as signed. For example, the signed byte representation of \fB-2\fP is \fB0xfe\fP, but \fB#06 #fe DIV\fP evaluates to \fB0x00\fP (\fB6 / 254 = 0\fP). For signed values the correct result should instead be \fB0xfd\fP (\fB6 / -2 = -3\fP).
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There is no \fIremainder\fP instruction, but the phrase \fBDIVk MUL SUB\fP can be used to compute the remainder.
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.SS AND ( x y -- x&y )
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Compute the bitwise union of the top two values of the stack.
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.SS ORA ( x y -- x|y )
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Compute the bitwise intersection of the top two values of the stack.
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.SS EOR ( x y -- x^y )
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Compute the bitwise exclusive-or (\fIxor\fP) of the top two values of the stack.
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.SS SFT ( x rl^ -- (x>>l)<<r )
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Compute a bit shift of the second value of the stack; the directions and distances are determined by the top value of the stack.
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Given a byte \fIrl\fP consisting of a low nibble (\fIl\fP) and a high nibble (\fIr\fP), this instruction shifts \fIx\fP left by \fIl\fP and then right by \fIr\fP.
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Right shifts are unsigned (they introduce zero bits). There are no signed shifts.
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For 16-bit (and 8-bit) values, one nibble (\fB0x0 - 0xf\fP) is sufficient to express all useful left or right shifts.
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Right: \fB#ff #03 SFT\fP evaluates to \fB0x1f\fP
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Left: \fB#ff #20 SFT\fP evaluates to \fB0xfc\fP
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Both: \fB#ff #23 SFT\fP evaluates to \fB0x7c\fP
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.SH SPECIAL INSTRUCTIONS
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These instructions do not accept all mode flags (some do not accept any).
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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 ( -- [pc*] ) jump to \fIaddr\fP saving the current address (\fIpc\fP) on \fIrst\fP
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(The instruction pointer will be moved forward 2 bytes, past the relative address.)
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These instructions are created by the assembler from special syntax:
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\fB!dest\fP produces \fBJMI wx yz\fP
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\fB?dest\fP produces \fBJCI wx yz\fP
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\fBdest\fP produces \fBJSI wx yz\fP (assuming \fBdest\fP is not a macro or reserved)
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.SS LIT, LIT2, LITr, and LIT2r
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Push a literal value on the stack.
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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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Literal values can be updated dynamically using store instructions:
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#abcd ;x STA2
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( later on... )
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LIT2 [ @x $2 ]
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.SH SEE ALSO
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https://wiki.xxiivv.com/site/uxntal_opcodes.html \fIUxntal Opcodes\fP
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https://wiki.xxiivv.com/site/uxntal_syntax.html \fIUxntal Syntax\fP
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https://wiki.xxiivv.com/site/uxntal_modes.html \fIUxntal Modes\fP
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https://wiki.xxiivv.com/site/uxntal_immediate.html \fIImmediate opcodes\fP
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https://wiki.xxiivv.com/site/varvara.html \fIVarvara\fP
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