How to get IDA Pro Free to automatically change segment on retf?

Question

I have been unable to get IDA pro to do a simple disassembly of a flat 16-bit binary (bootloader) file. I'm just getting started, and it seems to be harder than it should be.

There's a simple set of instructions, that do

seg000:7C5D                 push    7C0h
seg000:7C60                 pop     ds
seg000:7C61                 assume ds:nothing
seg000:7C61                 push    ds
seg000:7C62                 push    (offset unk_7C66 - offset loc_7C00)
seg000:7C65                 retf
seg000:7C65 ; --------------------------------------------------------------------------
seg000:7C66 unk_7C66        db  88h ; ê             ; DATA XREF: seg000:7C62o

Right under push 7c0h; pop ds it says assume ds:nothing. retf actually uses the values in the stack to set the segment and offset. Is there a way to manually enable such analysis? Are there more intelligent disassemblers out there? I'm having trouble following the flow and IDA isn't helping at ALL.

Answer 1

Apologies for bringing this up after half a year; hope the question is still relevant.

If this is occurring in multiple places, I would suggest to write a simple IDC script to solve the issue. The script would:

• Match the push/pop/push/push/retf instruction sequence;
• Set up a code cross-reference from the retf instruction to the calculated address;
• Place a comment;
• Mark the destination for analysis by IDA (optional, I think IDA should automatically start exploring after xref creation).

This should solve the issue at hand. Unfortunately, IDA is not as smart as we would like it to be.