Assembly fd意思

"FD" in assembly language is typically used as a mnemonic for the "Far Call" or "Far Jump" instruction on x86 processors. The "Far" prefix indicates that the call or jump is a segmented call, which means it can transfer control to a target address in a different code segment.

Here's what the "FD" instruction might mean in different contexts:

1. Far Call: When used with a call instruction, it specifies a far call, which is a call to a subroutine whose address is in a different code segment. This instruction stores the current segment and offset registers, and then loads the new segment and offset from the specified address.

2. Far Jump: Similarly, when used with a jump instruction, it specifies a far jump, which is a jump to a target address in a different code segment. This instruction immediately changes the program counter to the specified address, which can be in a different code segment.

Here's an example of a far call instruction in assembly language:

call fd 0x1234:0x5678

In this example, the call fd prefix is followed by the target address, which is a segment:offset pair. The 0x1234 is the segment address, and 0x5678 is the offset within that segment.

It's important to note that the use of segmented addressing and far calls/jumps has become less common with the introduction of protected mode and the flat memory model in modern x86 processors. In 32-bit protected mode and 64-bit mode, linear and virtual addressing are more commonly used, and far calls are typically not necessary.

If you encounter "FD" in a different context, it could have a different meaning depending on the specific assembly language or processor architecture you are working with. Always refer to the documentation or specifications for the specific architecture to understand the meaning of any mnemonic or instruction.