Set register to specific value for use in autoanalysis in IDA Pro 6.9

Question

I'm using IDA Pro 6.9 with some PowerPC disassembly. The code sets up r13 to a value, say 0x10000, then offsets that register to load and store memory in that region. r13 is never modified again in the code, it is only used for loading/storing data by offsetting.

e_stb     r7, -0x56E2(r13)

I'm hoping there is a way to tell IDA the value of r13 so that it will automatically generate a reference to the correct memory location so that I get something like:

e_stb     r7, -0x56E2(r13) # Named_Location

As well as the value at that location when I hover with the mouse.

Update:
Igor Skochinsky gave what is the correct answer but didn't fix my specific problem.

The answer seems to have worked anywhere r13 is used in an add instruction, e_add16i r31, r13, -0x2DF2 # unk_4000ADE2 but is not working for direct relative load/store instructions, e_stb r3, -0x2E08(r13) (no variable name, offset in red).

This might be a bug in IDA.

Answer 1

Just set it in processor options.

Answer 2

Regarding the potential bug in IDA:

First of all, you can add references yourself with idaapi.add_cref and idaapi.add_dref IDAPython APIs. This may be done with relatively simple script like this:

import idaapi
import idautils
import idc

predefined_r13 = YOUR_R13_VALUE

for h in idautils.Heads():
    dis = idc.GetDisasm(h) #probably there is another way to do it and parsing textual assembly is not the best option
    pos = dis.find ("(r13)")
    if pos != -1: #we have your register here
          splitted = dis[:pos].split() 
          lastarg = splitted[-1]
          lastarg_numeric = int(lastarg, 16)
          addr = predefined_r13 + lastarg_numeric
          idaapi.add_dref(h, addr, idc.dr_O)

Of course this script should be extended: the expression with r13 may be more complicated,the type of data reference may be different and it may be the code reference at all, and the problem is already solved for part of the cases as it is wrote in the answer by @Igor Skochinsky, but the general idea is illustrated pretty well.

In addition (if you want to override the work of disassembler) you can install your own disassembler module hook as @Rolf Rolles did in this article which will recompute all these things (you'll need to override idaapi.IDP_Hooks.custom_emu ), but adding data references is probably easier way to achieve your goal.