Wikifang:Network Translation Patchsite/BugVM

Bugsite contains a virtual machine which executes all (non-IRQ) game logic. It is a stack-based virtual machine with a large number of opcodes specific to the game.

Data and Link Stacks

Two stacks are managed by the VM: a link stack and a data stack. The link stack ($C100) supports up to $3F call frames of 4 bytes each, which is manipulated entirely by the call, jump-far, and return opcodes. The data stack ($C200) supports up to $55 data items of 3 bytes each. Each data item consists of a little-endian 16-bit word followed by a tag byte, which specifies if it's ($3D) an immediate value, ($1D) a word index into the indirect memory array ($C400), or ($1E) a bit index into the predicate array at 3:$D800. Data items are pushed onto the stack as immediate values and then cast to indirect or predicate offsets as necessary. Both stacks grow upwards from their base address.

Linkage Directory

Code and graphical resources exist entirely within sections, which can be referred to by their 16-bit linkage identifier. The linkage identifier indexes a directory starting from $A:$4000, with $800 8-byte directory entries per bank. In practice, while the directory could extend all the way across 8 banks, only 2 are used for the directory. (Effectively, linkages above $1000 are invalid and will horribly crash the game.) Directory entries appear to be stored in order of where their data is stored in ROM.

Each directory index contains a bank index, byte offset into that bank (0 means a pointer of $4000), and total size; with at least graphical resource loads capable of handling sections that span multiple banks. The remaining three bytes are padding and are always zero. There appear to be about 36 unused directory entries from linkage $FDC all pointing to $7F:$70FF with a size of zero.

Initial State

Execution of BugVM always starts from the beginning of linkage $0. Indirect and predicate memory is set to $0 upon game initialization.

Instruction Set

BugVM takes instructions as 8-bit opcodes which can optionally accept additional parameters. Most opcodes take arguments from the stack, rather than from the instruction stream. Native implementations for a particular opcode are referenced from the opcode table at $3E00, reproduced below:

	x0	x1	x2	x3	x4	x5	x6	x7	x8	x9	xA	xB	xC	xD	xE	xF
0x	NOP $5E9	NOP $5E9	NOP $5E9	??? $671	NOP $5E9	NOP $5E9	STR $68C	SUML $6C0	ANDL $6CA	OR $6D7	XOR $6E2	AND $6ED	CMP_EQ $6F8	CMP_NEQ $705	CMP_LEQ $712	CMP_LT $723
1x
2x
3x
4x
5x
6x
7x
8x
9x
Ax
Bx
Cx
Dx
Ex
Fx

NOP and NOP-alikes

BugVM contains a number of opcodes which do nothing, including both a single implementation of a NOP opcode used to fill in blank spots in the table as well as individual implementations which presumably did something in the past.

Opcode	Encoding(s)	Native Impl.	Operand Args	Stack Args	Description
NOP (Null OPeration)	Too many to list	ROM0:$5E9	None	None	Does nothing.

Boolean Logic & Comparison Operators

As convention for this table we treat zero as boolean TRUE and one as boolean FALSE. Other non-zero values are treated as FALSE, but boolean opcodes will not return nonstandard values.

Opcode	Encoding(s)	Native Impl.	Operand Args	Stack Args	Description
SUML (SUM Logical)	$07	$6C0	None	arg1, arg2 (TOP) -> bool (TOP)	Add arg1 and arg2. Push TRUE if result is zero, FALSE if non-zero.
ANDL (AND Logical)	$08	$6CA	None	arg1, arg2 (TOP) -> bool (TOP)	Bitwise-AND arg1 and arg2. Push TRUE if result is zero, FALSE if non-zero.
CMP_EQ (CoMParison EQual)	$0C	$6F8	None	arg1, arg2 (TOP) -> bool (TOP)	Compare arg1 and arg2. Push TRUE if both arguments are equal, FALSE otherwise.
CMP_NEQ (CoMParison Not EQual)	$0D	$705	None	arg1, arg2 (TOP) -> bool (TOP)	Compare arg1 and arg2. Push TRUE if both arguments are not equal, FALSE otherwise.
CMP_LEQ (CoMParison Less or EQual)	$0E	$712	None	arg1, arg2 (TOP) -> bool (TOP)	Compare arg1 and arg2. Push TRUE if arg1 is less than or equal to arg2, FALSE otherwise.
CMP_LT (CoMParison Less Than)	$0F	$723	None	arg1, arg2 (TOP) -> bool (TOP)	Compare arg1 and arg2. Push TRUE if arg1 is less than arg2, FALSE otherwise.

Bitwise logic

Opcode	Encoding(s)	Native Impl.	Operand Args	Stack Args	Description
OR (bitwise OR)	$09	$6D7	None	arg1, arg2 (TOP) -> value (TOP)	Bitwise-OR arg1 and arg2 as the return value.
XOR (bitwise eXclusive OR)	$0A	$6E2	None	arg1, arg2 (TOP) -> value (TOP)	Bitwise-XOR arg1 and arg2 as the return value.
AND (bitwise AND)	$0B	$6ED	None	arg1, arg2 (TOP) -> value (TOP)	Bitwise-AND arg1 and arg2 as the return value.

Memory & Stack Manipulation

Opcode	Encoding(s)	Native Impl.	Operand Args	Stack Args	Description
STR (SToRe)	$06	$68C	None	&address, value (TOP) -> (TOP)	Store the value at the memory location referenced by &address. The exact memory operations performed depend on the target of *address. If address is an indirect index, the memory location referenced by *address will be set to value. If address is a predicate index, the bit referenced by *address will be set or reset based on if value is zero or non-zero. It is illegal to STR into an immediate value, and doing so will cause the VM to halt operation.