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Chapter 11 Interfacing C and Assembly Code

Chapter 11 Interfacing C and Assembly Code. Learning Objectives. Different methods exist for interfacing C and assembly code: Calling assembly from C. Interrupt calling assembly routine. Intrinsics. Programming requirements when interfacing code. Introduction.

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Chapter 11 Interfacing C and Assembly Code

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  1. Chapter 11 Interfacing C and Assembly Code

  2. Learning Objectives • Different methods exist for interfacing C and assembly code: • Calling assembly from C. • Interrupt calling assembly routine. • Intrinsics. • Programming requirements when interfacing code.

  3. Introduction • This chapter shows how to interface C and assembly and how to use intrinsics. • As a general rule the code written in C is used for initialisation and for non-critical (in terms of speed or size) code. • Critical code (in terms of speed/size) can be written in assembly or linear assembly. • There are three different ways to interface C and assembly code: (1) C code call to the assembly function. (2) An interrupt can call an assembly function. (3) Call an assembly instruction using intrinsics.

  4. Calling Assembly from C main () { y = asmFunction (a, b); } • The C and assembly functions share the same resources (e.g. registers). • The C and assembly functions may exchange data. • Therefore code interfacing requires a means of handing-off data and control info and some rules of handling shared registers. _asmFunction b b3

  5. Calling Assembly from C main_c.c int asm_Function (short, short); short x = 0x4000, y = 0x2000; int z; void main (void) { z = asm_Function (x, y); } • Use “_” underscore in assembly for all variables or functions declared in C. • Labels also need to be global. asm_Function.c int asm_Function (short a, short b) { int y; y = (a * b) << 1; return y; } asm_Function.asm .global _asm_Function

  6. Passing Arguments between C and Assembly A B 0 1 2 3 ret addr 4 arg1/r_val arg2 5 6 arg3 arg4 7 8 arg5 arg6 9 10 arg7 arg8 11 12 arg9 arg10 13 14 15 • The following registers are used to pass and return variables when calling an assembly routine from C.

  7. Passing Arguments between C and Assembly A B 4 4 0x8000 0x4000 0x2000 0x2000 5 5 6 6 7 7 8 8 • Before assembly call. • After return from assembly call.

  8. Passing Arguments between C and Assembly Problem: • The C code will use some or all of the registers. • The assembly code may also require the use of some or all registers. • If nothing is done then on return to the C code some of the values may have been destroyed by the assembly code.

  9. Passing Arguments between C and Assembly A B Solution: • Both the C code and assembly code are responsible for saving some registers if they need to use them. 0 1 2 3 C code automatically saves these registers 4 5 6 7 8 9 10 Assembly code must save these registers - responsibility of the programmer 11 12 13 14 15

  10. Interfacing C and Assembly Examples • Setup code written in C and interfaced with critical code written in assembly can be found in the following chapters: • \Code\Chapter 14 - Finite Impulse Response Filters • \Code\Chapter 16 - Adaptive Filters • Setup code written in C and interfaced with critical code written in linear assembly can be found in the following chapters: • \Code\Chapter 15 - Infinite Impulse Response Filters • \Code\Chapter 17 - Goertzel Algorithm

  11. Chapter 11 Interfacing C and Assembly Code - End -

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