Intel 64 and IA-32 Architectures Software Developers Manual Volume 2B, Instruction Set Reference, N-Z

B-8 Vol. 2B
INSTRUCTION FORMATS AND ENCODINGS
B.1.4.8 Direction (d) Bit
In many two-operand instructions, a direction bit (d) indicates which operand is
considered the source and which is the destination. See Table B-11.
• When used for integer instructions, the d bit is located at bit 1 of a 1-byte primary
opcode. Note that this bit does not appear as the symbol “d” in Table B-13; the
actual encoding of the bit as 1 or 0 is given.
• When used for floating-point instructions (in Table B-16), the d bit is shown as bit
2 of the first byte of the primary opcode.
Table B-10. Encoding of Conditional Test (tttn) Field
t t t n Mnemonic Condition
0000 O Overflow
0001 NO No overflow
0010 B, NAE Below, Not above or equal
0011 NB, AE Not below, Above or equal
0100 E, Z Equal, Zero
0101 NE, NZ Not equal, Not zero
0110 BE, NA Below or equal, Not above
0111 NBE, A Not below or equal, Above
1000 S Sign
1001 NS Not sign
1010 P, PE Parity, Parity Even
1011 NP, PO Not parity, Parity Odd
1100 L, NGE Less than, Not greater than or equal to
1101 NL, GE Not less than, Greater than or equal to
1110 LE, NG Less than or equal to, Not greater than
1111 NLE, G Not less than or equal to, Greater than
Table B-11. Encoding of Operation Direction (d) Bit
d Source Destination
0 reg Field ModR/M or SIB Byte
1 ModR/M or SIB Byte reg Field