Expression Evaluator

The emulator is capable of evaluating expressions within breakpoint conditions and Go To prompts. These expressions inspect the current emulation state and can be used to dynamically process registers, variables and other data elements that may change from one invocation to the next.

Expressions are organized into series of tokens with three main modes of significance:

• Value	Numeric value that can be used in computation. This may take the form of a literal number provided directly in the expression, or one of the named symbols that represent things in the CPU state, current instruction and/or current memory access.
• Operator	Modifies one or two values through a given operation. Unary operators modify only the value on their right, while binary operators consider the values on their left and right and produce a new value as the result.
• Group	Operand groups, mainly through the use of parentheses `()`, override the default order of operations. Anything enclosed in a group is processed before any operations adjacent to that group.

Expressions are case-insensitive.

Values

Values come in two forms:

• Literal	Provided directly by the expression.
• Symbol	Refers to something in the current emulation state.

Values may be one of four data types:

• Signed Word	32-bit, two's complement integer.
• Unsigned Word	32-bit, unsigned integer.
• Float	32-bit floating short in IEEE-754 format.
• Boolean	1-bit value representing whether a condition is true or false.

Signed word and unsigned word are represented as sequences of character digits. A numeric token that begins with the characters 0x will be interpreted as hexadecimal, allowing letters A through F to be used as digits as well. If the given value can be represented in the signed word data type, the token is a signed word. Otherwise, if it can be represented in the unsigned word data type, the token is an unsigned word. If the value cannot be represented in either data type, a parsing error occurs.

Floats, like words, are represented as sequences of character digits. They are distinguished from words by the presence of a dot . character somewhere within the sequence. Only one dot may be present in a float literal, and dots cannot be used in hexadecimal literals. If the given value cannot be represented in the float data type, a parsing error occurs. Float values in the expression evaluator are subjected to the following restrictions: if the value of any float literal or the result of any float operation is NaN, an infinity, a denormal number or negative zero, it will be changed to positive zero.

Booleans are mechanically identical to signed words, but the semantics are slightly different. When used in a numeric operation, a boolean will use the value 0 if false or the value 1 if true. Boolean literals are specified with the named values true and false within the expression. In a boolean operation, any value that is zero is considered false, and any non-zero value is considered true.

Operators exist for converting between data types (see Operators below). A Go To expression will only be valid if its final data type is signed word or unsigned word.

The following symbols may be used for accessing information about the current instruction being executed:

address	Current memory access's address.
break	Identifies what type of break scenario is being considered. See below for a list of identifiers.
code	Current exception's exception code.
cond	Condition code for `Bcond` and `SETF` instructions. See below for a list of conditions.
disp	Displacement offset for jumps and memory accesses.
format	Current instruction's encoding format.
id	Identifies which specific instruction is being executed, considering `opcode` and, where applicable, `subopcode`. See below for a list of identifiers.
imm	Immediate operand.
opcode	Current instruction's top-level opcode.
reg1	Source register index.
reg2	Destination register index.
regid	System register index in `LDSR` and `STSR` instructions.
size	Number of bytes occupied by the current instruction.
subopcode	Current instruction's secondary opcode.
type	Data type of the current memory access.
value	Value read or to be written by the current memory access.
vector	Vector for `TRAP` instructions.

The following symbols may be used in conjunction with the break symbol:

exception

execute

read

write

The following symbols may be used in conjunction with the cond symbol:

The following symbols may be used in conjunction with the id symbol:

illegal

add_imm

add_reg

addf.s

addi

and

andbsu

andi

andnbsu

bcond

caxi

cli

cmp_imm

cmp_reg

cmpf.s

cvt.sw

cvt.ws

div

divf.s

divu

halt

in.b

in.h

in.w

jal

jmp

ld.b

ld.h

ld.w

ldsr

mov_imm

mov_reg

movbsu

movea

movhi

mpyhw

mul

mulf.s

mulu

not

notbsu

orbsu

ori

ornbsu

out.b

out.h

out.w

reti

rev

sar_imm

sar_reg

sch0bsd

sch0bsu

sch1bsd

sch1bsu

sei

setf

shl_imm

shl_reg

shr_imm

shr_reg

st.b

st.h

st.w

stsr

sub

subf.s

trap

trnc.sw

xor

xorbsu

xori

xornbsu

The following symbols may be used to retrieve the contents of a CPU program register:

r10

r11

r12

r13

r14

r15

r16

r17

r18

r19

r20

r21

r22

r23

r24

r25

r26

r27

r28

r29

r30

r31

The following symbols may be used to retrieve the contents of a CPU system register:

adtre

chcw

ecr

eipc

eipsw

fepc

fepsw

pir

psw

tkcw

sr29

sr30

sr31

Operators

Operators may apply to one (unary) or two (binary) values. All unary operators appear to the left of the value they modify (or another unary operator). Binary operators appear between the values they modify.

Each operator considers the types of its operands in order to produce a new value of the appropriate type. If the operands of a binary operator have different types, one of the values will be converted to the other type before performing the operation. The conversion selects the "greater" of the two types, in the following order (higher is "greater"):

↑	Float
	Unsigned word
	Signed word
↓	Boolean

For example, if an operation contains both a signed word and a float, the signed word value is converted to float before performing the operation.

Operators have assigned precedence that specifies the order of operations. For example, in the expression 1 + 2 * 3, the multiplication happens before the addition because it has higher precedence. Parentheses () may be used to encapsulate operations and guarantee that they are evaluated first regardless of the relative precedence of the adjacent operators. For example, in the expression (1 + 2) * 3, the addition happens before the multiplication because it is enclosed in parentheses.

The following operators may be used in expressions. Groups listed higher have higher precedence and happen before groups listed lower. Operators within groups have the same precedence and are processed according to the order in which they appear in the expression: right-to-left for unary operators, left-to-right for binary operators.

Unary	~	Not Bitwise	Cannot be used with a float value.
	!	Not Logical	Always produces a boolean.
	-	Negate	Cannot be used with an unsigned word value.
	bool	Cast to Boolean
	ceil	Round Up
	float	Cast to Float
	floor	Round Down
	round	Round to Nearest
	`s8`, `byte`	Cast to Signed Byte	Result is of type signed word.
	`s16`, `halfword`	Cast to Signed Halfword	Result is of type signed word.
	`s32`, `word`	Cast to Signed Word
	trunc	Truncate	Removes any fraction.
	`u8, ubyte`	Cast to Unsigned Byte	Result is of type unsigned word.
	`u16`, `uhalfword`	Cast to Unsigned Halfword	Result is of type unsigned word.
	`u32`, `uword`	Cast to Unsigned Word
	xfloat	Reinterpret as Float	The binary value is not modified.
	`xs32`, `xword`	Reinterpret as Signed Word	The binary value is not modified.
	`xu32`, `xuword`	Reinterpret as Unsgned Word	The binary value is not modified.
Binary	/	Divide	Zero divisor yields zero as result.
	*	Multiply
	%	Remainder	Zero divisor yields zero as result.
	+	Add
	-	Subtract
	<<	Shift Left	Cannot be used with float values.
	>>	Shift Right Arithmetic	Cannot be used with float values.
	>>>	Shift Right Logical	Cannot be used with float values.
	>	Greater	Always produces a boolean.
	>=	Greater or Equal	Always produces a boolean.
	<	Less	Always produces a boolean.
	<=	Less or Equal	Always produces a boolean.
	==	Equal	Always produces a boolean.
	!=	Not Equal	Always produces a boolean.
	&	And Bitwise	Cannot be used with float values.
	^	Exclusive Or Bitwise	Cannot be used with float values.
	\|	Or Bitwise	Cannot be used with float values.
	&&	And Logical	If left is true, returns right; else returns left.
	^^	Exclusive Or Logical	If only one operand is true, returns the truthy value.
	\|\|	Or Logical	If left is true, returns left; else returns right.

Memory Read

A value can be read from the memory bus of the emulation state. This is done by enclosing the part of the expression that represents the address in square brackets []. This functions in an identical manner to parentheses (), but will additionally perform the read operation.

Under most circumstances, a signed word read is performed. Certain operators can be placed in front of the group to alter how the read is performed:

• `float`	Behaves like `xfloat` instead.
• `s8`, `u8`	Performs a byte read of the specified signedness.
• `s16`, `u16`	Performs a halfword read of the specified signedness.

Reads are subjected to the alginment restrictions of the Virtual Boy's CPU: the lowest bit of the address is ignored for halfword reads, and the lowest two bits of the address are ignored for word reads.