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1. A frame stack haircut full#
2. A frame stack haircut code#
3. A frame stack haircut license#
4. A frame stack haircut free#

A push will decrement the stack pointer by 1 word or 4 bytes on a 32-bit ARM machine and store the value where the sp is pointing to.

A frame stack haircut full#

A frame stack haircut code#

This frame is then popped off the stack leaving us with the frame of 3 words that add has put on the stack.īefore the animation, the caller of add executed a bl add instruction which would store in the lr the instruction right after the C code of add(1,2) What you need to Know First We then restore r3 from the frame into r0 where the return of the function is stored.Īgain, with the function some_func another frame is created on the stack by this function call.

The frame gives us a way to protect what is local to our function when other functions are called since values can be stored outside of registers. If we didn’t have another function call in add, we would have no need to push lr onto the stack, and this is in fact what gcc will do if there isn’t another function call.ĭue to the function call some_func we are storing the value of r3 into the frame to protect if r3 is destroyed. This is because the some_func function call will end with a bx lr as it will push lr onto the stack. On the bl some_func instruction at 0x00010428 the lr will have the value 0x0001042c. Because this function call requires a bl we have to previously push lr onto the stack so that the program counter can be restored. We make another function call to some_func. Notice that the r3 register which is the result of c = a + b is stored into the frame.

Depending on the optimization and what is done the frame could be bigger and the arguments to the function int a, and int b could be stored in the frame. These 3 words hold a value for fp, lr and the local variable int c.

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Feel free to skip to the other example below for another explanation.Īs mentioned above, the arguments of the function are in registers r0 and r1 before the function is called. Stack of Frames in C and ARM Assembly Animation Details not shown in the Animation Watch the diagram for a couple of repetitions and then we’ll get into the theory, then explain some details this diagram leaves out. The right most diagram shows the stack and what is pushed onto the stack. Note, there are typically multiple instructions for one line of C code. As the C Code executes we can see the corresponding assembly code. In three columns we have C Code, corresponding ARM Assembly, and the Stack alongside one another. When the add function returns it will have the value 3 which will be stored in the r0 register, overwriting the first argument when the function was called. Before the add function is called, the caller, has stored the function arguments of 1 into r0, and the value 2 into r1. The animation below shows the execution of the C function add(1,2). We will disassemble C function calls to understand the stack of frames in ARM assembly.Īnimation of C code being Executed on an ARM Processor The stack frame also contains the previous frame pointer and program counter’s value to execute from once the frame is popped off the stack. This stack frame is used to allocate memory for local variables and intermediate values. Each function call creates a frame, and these frames are allocated on the stack. When functions are called they create the so-called stack of frames.

A frame stack haircut license#

The Stack of Frames in C with ARM Assembly Example | Lloyd Rochester's Geek Blog Lloyd Rochester's Geek Blog Home About License Navigateįunction calls in the C programming language make heavy use of the stack, also called the call stack.