[Vm-dev] [GSoC] Progress on the GdbARMPlugin

Lars lars.wassermann at googlemail.com
Mon Jul 9 19:28:17 UTC 2012

Hello squeak-dev,

in the last weeks, based on my simple code examples, I tried to copy the 
BochsIA32Plugin structures and adapt it to ARMulator/libopcodes. So far, 
I am able to run code sequentially with memory bounds checks. The 
(copied) test testExecutionTrap and testResetCPU are green. The 
testCallTrap might also be green, once I figured out what the 
correspondent of the IA32 call opcode in ARM is.

The code for library tests and the c-part of the plugin is accessible on 

For these tests, I implemented an alternative memory interface 
(armulmem.c). The problem with bounds checking and especially execution 
bounds was the prefetching emulation: Whenever an instruction is 
executed, the next two words are read from memory, too. Because of that, 
it is not possible to quit with the appropriate error code in the memory 
The current solution uses a custom Software Interrupt (0x200000). The 
memory interface returns the 'SWI 0x200000' opcode whenever it is asked 
for an instruction at an invalid address. This is implemented in 
ARMul_ReLoadInstr. For random memory access, the corresponding functions 
are GetWord and PutWord, which stop simulation immediately.

An interesting detail about the ARMulator is, that it is also the base 
for the ARM-Emulator used in Skyeye, beside being used in gdb. In case 
anybody wants to try using my code examples, it is very important to 
compile libsim.a with -DMODET (which is default on my system) or 
disabling that flag in GdbARMPlugin.h. That little flag took me at least 
two days, because it changes multiple offsets in the main struct 

What still is not included on that site is reporting. The Bochs test 
cases show that there should be string-errors. The ARMulator does not 
provide those strings, and since so far this is the only error, I put no 
additional effort into that.

The next step is getting the testNfib* green. These compute the 
Fibonacci function for different arguments. For that, I need to 
understand the ARM ABI[2].

I'd be grateful if anybody can recommend a more abstract source for the 
Enjoy your evening,

[1] https://github.com/mas7ermind/gdb-arm-tests

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