3Com 4007, Implementation Manual, Page: 327 / 684

The Packet Filtering Language 327
ge (greater than or
equal to) 1 byte Pops two values from the stack and performs an
unsigned comparison. If the first is greater than or
equal to the second, a byte containing the non-zero
value is pushed onto the stack; otherwise, a byte
containing 0 is pushed. The contents of the stack
determine the size of the operands.
and (bit-wise AND) 1 byte Pops two values from the stack and pushes the bit-wise
AND
of these values back onto the stack. The contents
of the stack determine the size of the operands and the
result.
This is a bit-wise operator. Each bit of the operands is
logically compared to produce the resulting bit
or (bit-wise OR) 1 byte Pops two values from the stack and pushes the bit-wise
OR of these values back onto the stack. The contents
of the stack determine the operand size and the result.
This is a bit-wise operator. Each bit of the operands is
logically compared to produce the resulting bit
xor (bit-wise
exclusive-OR) 1 byte Pops two values from the stack and pushes the bit-wise
exclusive
- OR of these values back onto the stack. The
contents of the stack determines the operand size and
the result.
This is a bit-wise operator. Each bit of the operands is
logically compared to produce the resulting bit
not 1 byte Pops a byte from the stack; if its value is non-zero, a
byte containing 0 is pushed back onto the stack.
Otherwise, a byte containing the value is pushed back
onto the stack.
accept 1 byte Conditionally accepts the packet that is being
examined. Pops a byte from the stack. If its value is
non-zero, the packet is accepted and evaluation of the
filter ends immediately; otherwise, filter evaluation
continues with the next instruction.
Use
accept
with
and
and
or
operators when you
have sequential tests and you would like the filter to
accept a packet before the entire expression has been
evaluated. Using
accept
can significantly improve the
performance of certain types of filters. See
“Implementing Sequential Tests in a Packet Filter”
elsewhere in the chapter for more information.
Table 66 Packet Filtering Opcodes (continued)
Opcode
Memory
Requirements
Description