Re: [patch] disassembly window

[Andrew Cagney <cagney@redhat.com>]

Zhao Shujing wrote:
> Hi
>
> This patch is to fix bug #4932 and other bugs of disassembly window. 
> rowPrepend is added to prepend rows by calculating memory information
> like rowAppend. Because the methods that are provided by class
> Disassembler, disassembleInstructions and
> disassembleInstructionsStartEnd, can only read the instructions that are
> following some address, rowPrepend have to use two while execution
> control to read the instructions that are preceding some address.
> Any suggestions are welcomed.
>   

Pearly, nice work.

The challenge here, and the reason why the disassembler only goes 
forward from PC is that, in general, it isn't possible to disassemble 
backwards.  This is because architectures such as the i386 and x86-64 
have variable length instructions making it effectively impossible to 
figure out where, looking backwards, an instruction starts.  For 
instance, looking backwards is that a one byte int80 instruction, or a 
multi-byte instruction loading the hex code for int80?

For disassembling a range, can I suggest doing something similar to the 
disassembler command in frysk.hpd.DisassemblerCommand.  That code first 
attempts to fetch the frysk.symtab.Symbol at the frame's 
adjusted-address and then uses its address/size to determine the start 
address and size to disassemble.  If the symbol isn't available, then 
I'd just disassemble a small range from the frame's PC.

One heads up for you; you may want to consider configuring your personal 
build with --with-libopcodes.

Andrew