Electronic – How to decode ARMv7 instructions

armarm7assembly

I'm trying to understand ARM instruction encoding.

The following image shows C code along with its corresponding assembly instructions:

I looked at the ARMv7 reference manual. However, I couldn't find the relationship between the machine code and the corresponding assembly instruction.

For example,

I couldn't relate the machine code

F1AD0D08

to the assembly instruction

sub.w sp, sp, #8

Could you help me with this?

======================================================

Based on the answers and after looking at the reference manual, I added the following information.

The processor name is Cortex-M4F.

The processor architecture of the microcontroller I use is ARMv7.

Specifically, the microcontroller profile employed is ARMv7-M.

From the manual:

ARMv7-M The microcontroller profile for systems supporting only the
Thumb instruction set, and where overall size and deterministic
operation for an implementation are more important than absolute
performance.

Best Answer

The ARMv7-M reference manual does not have a decode lookup table, but the encoding for each instruction is listed.

In the specific manual you have linked, you will find this in the section A6.7 - Alphabetical list of ARMv7-M Thumb instruction.

This contains - as stated - a list of every instruction, and their corresponding binary encoding. For example, page A6-16 has the binary encoding for the ADC (immediate) instruction. We can see that it starts with 11110... and spans two words.

The problem is going "backwards". The manual does indeed contain all the necessary information, but it is difficult to search. The disassembler has the data structures in place to do that for you, and if you want a table and do it manually, I suggest looking at the source code files for an ARM disassembler, for example GNU binutils.

Related Solutions

Programming microcontrollers in ASM or C & how it’s done

A compiler can (for the purpose of your question) be divided in two parts:

The first part reads your code, analyzes it and checks it for errors, and does all kinds of clever things with it that have no relation with the target computer
The second part takes the output of the first part and translates it into a sequence of machine instructions (either in textual form, or in binary form, or both).

The second part is specific for the CPU on which your program must run. If you want to run on a PIC 16F, you must have a compiler that generates machine instructions for that specific chip, and often the compiler will need to know the exact chip for which you are compiling.

For an AVR, ARM, Cortex, MIPs, etc CPU you will need a different compiler. There are compilers that contain the second part (the back-end) for more than one target. In that case you must somehow specify which of the back-ends must be used.

Electronic – Tiny Code for dumping Flash Memory

I'm not used to program for ARM processors and don't know which compiler you use, so maybe the proposed changes does nothing at all, but hey, at least let's try!

1-Inline your functions:

A good compiler should already inline a function if optimizations are well set, but it's worth to inline it so you remove the call's and ret's

2-Avoid branching:

In some architectures an IF can be translated in: load, test, branch, three instructions, if you can do it without branching it can use less instructions.

So, the proposed code is:

    int main() 
    {
      // start dumping at memory address 0x08000000
      unsigned int *p;
      int i;

      p = (uint32_t *)(0x08000000u);

      // configure pin 1.0 and pin 1.1 as output (push-pull)
      PORT1->IOCR0 = 0x8080UL;

      do
      {
        for (i = 0; i < 32; i++)
        {
          // set pin 1.1 to low (SPI clock)
          PORT1->OUT &= (~0x2UL);
          PORT1->OUT = (PORT1->OUT & 0xFFFE) | (data & 0x01)
          PORT1->OUT |= 0x2UL;
          data >>= 1;
        }

      }while(p++)
    }

Give it a try and comment the results.

Best Answer

Related Solutions

Programming microcontrollers in ASM or C & how it’s done

Electronic – Tiny Code for dumping Flash Memory

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