Volume 3: General-Purpose and System Instructions - Stanford ...

24594 Rev. 3.10 February 2005 AMD64 TechnologyOUTSOUTSBOUTSWOUTSDOutput StringCopies data from the memory location pointed to by DS:rSI to the I/O port address(0000h to FFFFh) specified in the DX register, and then increments or decrements therSI register according to the setting of the DF flag in the rFLAGS register.If the DF flag is 0, the instruction increments rSI; otherwise, it decrements rSI. Itincrements or decrements the pointer by 1, 2, or 4, depending on the size of the valuebeing copied.The OUTSx instruction uses an explicit memory operand (second operand) todetermine the type (size) of the value being copied, but always uses DS:rSI for thelocation of the value to copy. The explicit register operand specifies the I/O portaddress and must always be DX.The no-operands forms of the instruction use the DS:[rSI] register pair to point to thedata to be copied and the DX register as the destination. The mnemonic specifies thesize of the I/O port and the type (size) of the value being copied.The OUTSx instruction supports the REP prefix. For details about the REP prefix, see“Repeat Prefixes” on page 10.If the operand size is 64-bits, OUTS only writes to a 32-bit I/O port.If the CPL is higher than the IOPL or the mode is virtual mode, OUTSx checks the I/Opermission bitmap in the TSS before allowing access to the I/O port. See Volume 2 fordetails on the TSS I/O permission bitmap.Mnemonic Opcode DescriptionOUTS DX, mem8OUTS DX, mem16OUTS DX, mem326E6F6FOutput the byte in DS:rSI to the port specified in DX, thenincrement or decrement rSI.Output the word in DS:rSI to the port specified in DX, thenincrement or decrement rSI.Output the doubleword in DS:rSI to the port specified in DX, thenincrement or decrement rSI.OUTSx 221