Debugging with GDB: Bytecode Descriptions

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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.

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