CODEWITHSUNDEEP
assemblywindows-xpx86x86-16
0x90
0x90

Reputation: 40982

What is interrupt 69 in x86 16-bit assembly language?

I was looking at some snippet and saw these lines (part of the .com file):

  DB 66; 
  CALL D59C:C2C0; 
  INT 69
  MOV SI, C8C6

What does INT 69 do?

I didn't find anything online, and I didn't find anything here as well:

The weird thing is that there is no moving any value to AH or AL before the INT 69.

EMU8086. 8086 microprocessor emulator. Integrated disassembler.

Upvotes: 0

Views: 766

Answers (3)

Alexey Frunze
Alexey Frunze

Reputation: 62048

Assuming that the given code is meant to execute in 16-bit mode and encodes consecutive instructions, I have a different interpretation of it.

DB 66 is the operand size prefix. In 16-bit mode it tells the CPU to interpret instruction operands as 32-bit instead of 16-bit. So, the CALL instruction will be interpreted as CALL 16-bit selector:32-bit offset instead of CALL 16-bit selector:16-bit offset. The "missing" 2 bytes of the address are the INT 69 "instruction".

The effective code is then this:

CALL 69CD:D59CC2C0
MOV SI, C8C6

But that doesn't make much sense to me because a call with such an address (offset > 0FFFFh) will cause an exception. What kind of code is that?

Upvotes: 3

Matthew Slattery
Matthew Slattery

Reputation: 46998

Assuming that everything is in hex here, if this is 16-bit code then we have:

66                         DB 66
9A C0 C2 9C D5             CALL D59C:C2C0
CD 69                      INT 69
BE C6 C8                   MOV SI, C8C6

But 0x66 is the operand size override prefix (presumably just not disassembled properly here), which (in 16-bit code) causes the following instruction to take a 32-bit operand instead of a 16-bit one. So this code is actually:

66 9A C0 C2 9C D5 CD 69    CALL 69CD:D59CC2C0
BE C6 C8                   MOV SI, C8C6

A far call to a fairly random-looking 16:32 bit absolute address from 16-bit code doesn't look very plausible to me.

So I would guess that this is actually data, rather than code...

Upvotes: 9

Bob Kaufman
Bob Kaufman

Reputation: 12815

A search for x86 interrupts yielded this list, presumably written by one Ralf Brown. If it's what I believe it is, this is the definitive list of interrupts from a generation ago. Brings back memories.

Upvotes: 2

Related Questions