A-8 APPENDIX A: PACKET FILTER OPCODES, EXAMPLES, AND SYNTAX ERRORS

reject

Description:

Conditionally rejects the packet being examined. A byte is popped from the stack. If it is non-zero, the packet is rejected and evaluation of the filter ends immediately; otherwise, filter evaluation continues with the next instruction.

Storage Needed:

1 byte

shiftl (shift left)

Description:

Pops two values from the stack and shifts the first operand left by the number of bits specified by the second operand. Bits shifted out of the left side of the operand are discarded and zeros are shifted in from the right. The resulting value is pushed back onto the stack. The size of the first operand and the size of the result are determined by the contents of the top of the stack. The second operand is always 1 byte and only the low 5 bits of the byte are used as the shift count.

Storage Needed:

1 byte

shiftr (shift right)

Description:

Pops two values from the stack and shifts the first operand right by the number of bits specified by the second operand. Bits shifted out of the right side of the operand are discarded and zeros are shifted in from the left. The resulting value is pushed back onto the stack. The size of the first operand and the size of the result are determined by the contents of the top of the stack. The second operand is always 1 byte and only the low 5 bits of the byte are used as the shift count.

Storage Needed:

1 byte

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3Com 2200 manual Reject, Byte Shiftl shift left, Byte Shiftr shift right