gdb: Bytecode Descriptions
F.2 Bytecode Descriptions
=========================
Each bytecode description has the following form:
'add' (0x02): A B => A+B
Pop the top two stack items, A and B, as integers; push their sum,
as an integer.
In this example, 'add' is the name of the bytecode, and '(0x02)' is
the one-byte value used to encode the bytecode, in hexadecimal. The
phrase "A B => A+B" shows the stack before and after the bytecode
executes. Beforehand, the stack must contain at least two values, A and
B; since the top of the stack is to the right, B is on the top of the
stack, and A is underneath it. After execution, the bytecode will have
popped A and B from the stack, and replaced them with a single value,
A+B. There may be other values on the stack below those shown, but the
bytecode affects only those shown.
Here is another example:
'const8' (0x22) N: => N
Push the 8-bit integer constant N on the stack, without sign
extension.
In this example, the bytecode 'const8' takes an operand N directly
from the bytecode stream; the operand follows the 'const8' bytecode
itself. We write any such operands immediately after the name of the
bytecode, before the colon, and describe the exact encoding of the
operand in the bytecode stream in the body of the bytecode description.
For the 'const8' bytecode, there are no stack items given before the
=>; this simply means that the bytecode consumes no values from the
stack. If a bytecode consumes no values, or produces no values, the
list on either side of the => may be empty.
If a value is written as A, B, or N, then the bytecode treats it as
an integer. If a value is written is ADDR, then the bytecode treats it
as an address.
We do not fully describe the floating point operations here; although
this design can be extended in a clean way to handle floating point
values, they are not of immediate interest to the customer, so we avoid
describing them, to save time.
'float' (0x01): =>
Prefix for floating-point bytecodes. Not implemented yet.
'add' (0x02): A B => A+B
Pop two integers from the stack, and push their sum, as an integer.
'sub' (0x03): A B => A-B
Pop two integers from the stack, subtract the top value from the
next-to-top value, and push the difference.
'mul' (0x04): A B => A*B
Pop two integers from the stack, multiply them, and push the
product on the stack. Note that, when one multiplies two N-bit
numbers yielding another N-bit number, it is irrelevant whether the
numbers are signed or not; the results are the same.
'div_signed' (0x05): A B => A/B
Pop two signed integers from the stack; divide the next-to-top
value by the top value, and push the quotient. If the divisor is
zero, terminate with an error.
'div_unsigned' (0x06): A B => A/B
Pop two unsigned integers from the stack; divide the next-to-top
value by the top value, and push the quotient. If the divisor is
zero, terminate with an error.
'rem_signed' (0x07): A B => A MODULO B
Pop two signed integers from the stack; divide the next-to-top
value by the top value, and push the remainder. If the divisor is
zero, terminate with an error.
'rem_unsigned' (0x08): A B => A MODULO B
Pop two unsigned integers from the stack; divide the next-to-top
value by the top value, and push the remainder. If the divisor is
zero, terminate with an error.
'lsh' (0x09): A B => A<<B
Pop two integers from the stack; let A be the next-to-top value,
and B be the top value. Shift A left by B bits, and push the
result.
'rsh_signed' (0x0a): A B => '(signed)'A>>B
Pop two integers from the stack; let A be the next-to-top value,
and B be the top value. Shift A right by B bits, inserting copies
of the top bit at the high end, and push the result.
'rsh_unsigned' (0x0b): A B => A>>B
Pop two integers from the stack; let A be the next-to-top value,
and B be the top value. Shift A right by B bits, inserting zero
bits at the high end, and push the result.
'log_not' (0x0e): A => !A
Pop an integer from the stack; if it is zero, push the value one;
otherwise, push the value zero.
'bit_and' (0x0f): A B => A&B
Pop two integers from the stack, and push their bitwise 'and'.
'bit_or' (0x10): A B => A|B
Pop two integers from the stack, and push their bitwise 'or'.
'bit_xor' (0x11): A B => A^B
Pop two integers from the stack, and push their bitwise
exclusive-'or'.
'bit_not' (0x12): A => ~A
Pop an integer from the stack, and push its bitwise complement.
'equal' (0x13): A B => A=B
Pop two integers from the stack; if they are equal, push the value
one; otherwise, push the value zero.
'less_signed' (0x14): A B => A<B
Pop two signed integers from the stack; if the next-to-top value is
less than the top value, push the value one; otherwise, push the
value zero.
'less_unsigned' (0x15): A B => A<B
Pop two unsigned integers from the stack; if the next-to-top value
is less than the top value, push the value one; otherwise, push the
value zero.
'ext' (0x16) N: A => A, sign-extended from N bits
Pop an unsigned value from the stack; treating it as an N-bit
twos-complement value, extend it to full length. This means that
all bits to the left of bit N-1 (where the least significant bit is
bit 0) are set to the value of bit N-1. Note that N may be larger
than or equal to the width of the stack elements of the bytecode
engine; in this case, the bytecode should have no effect.
The number of source bits to preserve, N, is encoded as a single
byte unsigned integer following the 'ext' bytecode.
'zero_ext' (0x2a) N: A => A, zero-extended from N bits
Pop an unsigned value from the stack; zero all but the bottom N
bits.
The number of source bits to preserve, N, is encoded as a single
byte unsigned integer following the 'zero_ext' bytecode.
'ref8' (0x17): ADDR => A
'ref16' (0x18): ADDR => A
'ref32' (0x19): ADDR => A
'ref64' (0x1a): ADDR => A
Pop an address ADDR from the stack. For bytecode 'ref'N, fetch an
N-bit value from ADDR, using the natural target endianness. Push
the fetched value as an unsigned integer.
Note that ADDR may not be aligned in any particular way; the 'refN'
bytecodes should operate correctly for any address.
If attempting to access memory at ADDR would cause a processor
exception of some sort, terminate with an error.
'ref_float' (0x1b): ADDR => D
'ref_double' (0x1c): ADDR => D
'ref_long_double' (0x1d): ADDR => D
'l_to_d' (0x1e): A => D
'd_to_l' (0x1f): D => A
Not implemented yet.
'dup' (0x28): A => A A
Push another copy of the stack's top element.
'swap' (0x2b): A B => B A
Exchange the top two items on the stack.
'pop' (0x29): A =>
Discard the top value on the stack.
'pick' (0x32) N: A ... B => A ... B A
Duplicate an item from the stack and push it on the top of the
stack. N, a single byte, indicates the stack item to copy. If N
is zero, this is the same as 'dup'; if N is one, it copies the item
under the top item, etc. If N exceeds the number of items on the
stack, terminate with an error.
'rot' (0x33): A B C => C A B
Rotate the top three items on the stack. The top item (c) becomes
the third item, the next-to-top item (b) becomes the top item and
the third item (a) from the top becomes the next-to-top item.
'if_goto' (0x20) OFFSET: A =>
Pop an integer off the stack; if it is non-zero, branch to the
given offset in the bytecode string. Otherwise, continue to the
next instruction in the bytecode stream. In other words, if A is
non-zero, set the 'pc' register to 'start' + OFFSET. Thus, an
offset of zero denotes the beginning of the expression.
The OFFSET is stored as a sixteen-bit unsigned value, stored
immediately following the 'if_goto' bytecode. It is always stored
most significant byte first, regardless of the target's normal
endianness. The offset is not guaranteed to fall at any particular
alignment within the bytecode stream; thus, on machines where
fetching a 16-bit on an unaligned address raises an exception, you
should fetch the offset one byte at a time.
'goto' (0x21) OFFSET: =>
Branch unconditionally to OFFSET; in other words, set the 'pc'
register to 'start' + OFFSET.
The offset is stored in the same way as for the 'if_goto' bytecode.
'const8' (0x22) N: => N
'const16' (0x23) N: => N
'const32' (0x24) N: => N
'const64' (0x25) N: => N
Push the integer constant N on the stack, without sign extension.
To produce a small negative value, push a small twos-complement
value, and then sign-extend it using the 'ext' bytecode.
The constant N is stored in the appropriate number of bytes
following the 'const'B bytecode. The constant N is always stored
most significant byte first, regardless of the target's normal
endianness. The constant is not guaranteed to fall at any
particular alignment within the bytecode stream; thus, on machines
where fetching a 16-bit on an unaligned address raises an
exception, you should fetch N one byte at a time.
'reg' (0x26) N: => A
Push the value of register number N, without sign extension. The
registers are numbered following GDB's conventions.
The register number N is encoded as a 16-bit unsigned integer
immediately following the 'reg' bytecode. It is always stored most
significant byte first, regardless of the target's normal
endianness. The register number is not guaranteed to fall at any
particular alignment within the bytecode stream; thus, on machines
where fetching a 16-bit on an unaligned address raises an
exception, you should fetch the register number one byte at a time.
'getv' (0x2c) N: => V
Push the value of trace state variable number N, without sign
extension.
The variable number N is encoded as a 16-bit unsigned integer
immediately following the 'getv' bytecode. It is always stored
most significant byte first, regardless of the target's normal
endianness. The variable number is not guaranteed to fall at any
particular alignment within the bytecode stream; thus, on machines
where fetching a 16-bit on an unaligned address raises an
exception, you should fetch the register number one byte at a time.
'setv' (0x2d) N: V => V
Set trace state variable number N to the value found on the top of
the stack. The stack is unchanged, so that the value is readily
available if the assignment is part of a larger expression. The
handling of N is as described for 'getv'.
'trace' (0x0c): ADDR SIZE =>
Record the contents of the SIZE bytes at ADDR in a trace buffer,
for later retrieval by GDB.
'trace_quick' (0x0d) SIZE: ADDR => ADDR
Record the contents of the SIZE bytes at ADDR in a trace buffer,
for later retrieval by GDB. SIZE is a single byte unsigned integer
following the 'trace' opcode.
This bytecode is equivalent to the sequence 'dup const8 SIZE
trace', but we provide it anyway to save space in bytecode strings.
'trace16' (0x30) SIZE: ADDR => ADDR
Identical to trace_quick, except that SIZE is a 16-bit big-endian
unsigned integer, not a single byte. This should probably have
been named 'trace_quick16', for consistency.
'tracev' (0x2e) N: => A
Record the value of trace state variable number N in the trace
buffer. The handling of N is as described for 'getv'.
'tracenz' (0x2f) ADDR SIZE =>
Record the bytes at ADDR in a trace buffer, for later retrieval by
GDB. Stop at either the first zero byte, or when SIZE bytes have
been recorded, whichever occurs first.
'printf' (0x34) NUMARGS STRING =>
Do a formatted print, in the style of the C function 'printf').
The value of NUMARGS is the number of arguments to expect on the
stack, while STRING is the format string, prefixed with a two-byte
length. The last byte of the string must be zero, and is included
in the length. The format string includes escaped sequences just
as it appears in C source, so for instance the format string
'"\t%d\n"' is six characters long, and the output will consist of a
tab character, a decimal number, and a newline. At the top of the
stack, above the values to be printed, this bytecode will pop a
"function" and "channel". If the function is nonzero, then the
target may treat it as a function and call it, passing the channel
as a first argument, as with the C function 'fprintf'. If the
function is zero, then the target may simply call a standard
formatted print function of its choice. In all, this bytecode pops
2 + NUMARGS stack elements, and pushes nothing.
'end' (0x27): =>
Stop executing bytecode; the result should be the top element of
the stack. If the purpose of the expression was to compute an
lvalue or a range of memory, then the next-to-top of the stack is
the lvalue's address, and the top of the stack is the lvalue's
size, in bytes.
Info Catalog
gdb: General Bytecode Design
gdb: Agent Expressions
gdb: Using Agent Expressions