In Volume 2, CHAPTER 4 INSTRUCTION SET REFERENCE, M-U, the description of OUT Instruction has this sentence:

"using the DX register as a source operand allows I/O ports from 0 to 65,535 to be accessed."

I think when using OUT instruction, the DX register is specify the IO port not the source operand.

CodePudding user response：

What the manual attempts to say is true, even if the wording is a bit unclear.

When using an immediate parameter to specify the port number (like OUT imm8, AX), you are limited to ports 0-255 as the immediate operand is 8 bits.

When using DX to specify the port number (like OUT DX, EAX), all 65536 ports can be used.

The term 'source operand' is a bit misleading, as in the above example, the contents of EAX are output to the port specified by DX. So, the contents of DX specify the destination port number.