Why do call, interrupt, and trap gates have two offset fields?

[chrstrbrts]

Hi guys,

Why do call gates, interrupt gates, and trap gates have two offset fields?

My theory is that you have the option of jumping to two different procedures within the same segment.

What do you guys think?

 

[chrstrbrts]

Also, how does the computer know which offset to use?

There's only one segment selector which makes sense, but with more than one offset I don't see how the system knows which address to jump to.

 

[Venix]

They're combined to create a single 32 bit offset value.

The descriptor was designed when x86 CPUs were 16 bits, so it had a 16 bit segment selector and 16 bit offset. 32 bit protected mode requires a 32 bit offset, so a second 16 bit field was added. The original offset field contains the low 16 bits of the 32 bit offset, and the new field the high 16 bits. amd64 adds another field for the high 32 bits of a 64 bit offset.

 

[chrstrbrts]

They're combined to create a single 32 bit offset value.

The descriptor was designed when x86 CPUs were 16 bits, so it had a 16 bit segment selector and 16 bit offset. 32 bit protected mode requires a 32 bit offset, so a second 16 bit field was added. The original offset field contains the low 16 bits of the 32 bit offset, and the new field the high 16 bits. amd64 adds another field for the high 32 bits of a 64 bit offset.

Of course!

Makes perfect sense.

Thank you, sir.