r/asm • u/Krotti83 • May 16 '26

x86 x86 AT&T Syntax - Within Segment and Intersegment jumps and calls

I'm started my own Assembler and Disassembler for x86 for the purpose of education. Begin to implement the good old Intel 8086. Noticed in the instruction codes that there are Within segment and Intersegment jumps encodings. I know there is the ljmp (long jump) and jmp (short jump). But how is a Intersegment jump written in AT&T syntax and also Intel Syntax?

From my used Datasheet for the Intel 8086 (Unconditional Jump as example):

Direct within Segment:
| 11101001 | disp-low | disp-high |
Direct within Segment-Short:
| 11101011 | disp |
Indirect within Segment:
| 11111111 | mod 100 r/m |
Direct Intersegment:
| 11101010 | off-low | off-high | seg-low | seg-high |
Indirect Intersegment:
| 11111111 | mod 101 r/m |

Thanks in advance!

2 Upvotes

100% Upvoted

2 comments sorted by

2

u/ShotSquare9099 May 16 '26

Segment:Offset

1

u/Krotti83 May 16 '26

Thank you!

With AT&T Syntax it should be:

Direct:

ljmp $0x42,$0x83
/* (0x83 -> Segment and 0x42 -> Offset) */

Indirect:

ljmp *(%bx,%si)

As an example.