CE-2810 Dr. Mark L. Hornick 1 Mixing C and assembly Safety goggles on!

CE-2810 Dr. Mark L. Hornick 2 Prelude: How do arguments get passed into a function when that function is called from C ? And how do return values get passed back to the calling function from the called function?

CS-280 Dr. Mark L. Hornick 3 What do compilers do? C/C++ compilers generally Pass parameters using a register/stack combination Return values via registers Save registers at the beginning of a function Restore registers before the function returns

CE-2810 Dr. Mark L. Hornick 4 Passing arguments among C functions Every C compiler has it’s own rules and conventions for passing arguments to functions Arguments can be passed 1. via Registers 2. via the Stack 3. Both of the above

CE-2810 Dr. Mark L. Hornick 5 GCC for Atmega32 uses Registers as much as possible to pass arguments Consider a function: int16_t add( uint8_t x, uint16_t y); uint8_t means “unsigned 8-bit int” uint16_t means “unsigned 16-bit int” int16_t means “signed 16-bit int” (same as int on Atmega32) GNU GCC passes arguments left to right using registers r25 to r8 All arguments always take an even number of registers All arguments always take an even number of registers Above, value of x (single byte) is placed in r24:r25 (low, high bytes)  The value of x is placed in r24  The value of 0 is placed in r25 Value of y (2 bytes) is placed in r22:r23  The low byte value of y is placed in r22  The high byte value of y is placed in r23 Note that r26:r31 are not used for passing arguments or return values since these registers are also the X, Y, and Z registers which the compiler uses for other purposes What about the return value??

CE-2810 Dr. Mark L. Hornick 6 GCC convention for returning values from functions Return values always take an even number of registers 8-bit values are returned in r24:r25 Value in r24 0 in r25 16-bit values returned in r24:r25 Low byte in r24 High byte in r25 32-bit values returned in r22:r25 Lowest byte in r22 Highest byte in r25 64-bit values returned in r18:r25 lowest byte in lowest register

CE-2810 Dr. Mark L. Hornick 7 Example of 2-byte argument passing Consider the function: int add( int x, int y); Size of int in C is dependent on processor On Atmega32, int is 2 bytes  Within MSVS for your PC, int means 4 bytes  Note: Java int is always 4 bytes GNU GCC passes arguments left to right using registers r25 to r8 All arguments always take an even number of registers Above, value of x (two bytes) is placed in r24:r25 (low, high bytes) Value of y is placed in r23-r22 The return value is passed back in r24:r25 (low, high)

CE-2810 Dr. Mark L. Hornick 8 Example of 4-byte argument passing Consider the function: long add(int x, long y); Size of long is 4 bytes GNU GCC passes arguments left to right using registers r25 to r8 Above, value of x (2 bytes) is placed in r24:r25  r24 is low byte; r25 is high byte Value of y is placed in r20:r23  r20 is lowest byte; r23 is highest byte The return value is passed back in r22:r25 r22 is lowest byte

CE-2810 Dr. Mark L. Hornick 9 What happens when we run out of registers to pass arguments? Registers r25 to r8 can accommodate only 9 two-byte arguments 18 bytes total via registers Consider the function: long add( long a, long b, long c, long d, long e); 20 bytes worth of arguments in this case

CE-2810 Dr. Mark L. Hornick 10 When it runs out of Registers, the compiler uses the Stack to pass additional arguments long add( long a, long b, long c, long d, long e); 20 bytes worth of arguments in this case, but only 18 1-byte registers are available Approach: Arguments a, b, c, d (16 bytes total) are passed using registers r25 to r10 r22:r25 for a, r18:r21 for b The remaining argument e (4 bytes) is passed on the Stack Leaving registers r8, r9 unused – the compiler does not split an argument between Stack and Registers Return addr (high) Return addr (low) e1 (low) e2 e3 e4 (high) ??? SP

CE-2810 Dr. Mark L. Hornick 11 What do you think would happen here? Consider the function: long add( long a, long b, long c, long d, int e, int f); Also 20 bytes worth of arguments in this case

CE-2810 Dr. Mark L. Hornick 12 The called subroutine must find the value of e on the Stack behind the return address Example: long add( long a, long b, long c, long d, long e); Parameters a, b, c, d (16 bytes) are passed using registers r25 to r10 Remaining parameter e is passed as 4 bytes on the Stack in the calling code: in ZH, SPH; load SP to Z in ZL, SPL ; Z points to cell containing ??? now. ldd temp, Z+3; load e1 to temp ldd temp, Z+4; load e2 to temp... Return addr (high) Return addr (low) e1 (low) e2 e3 e4 (high) ??? SP

CE-2810 Dr. Mark L. Hornick 13 General rules of register usage in mixed C/ASM These rules apply when you write assembly subroutines that you’re mixing with compiler-generated code R0 – used by compiler as a temporary register If you use R0 and then call code generated by the compiler, that code may overwrite R0 R0 is not saved or restored by compiler-generated C code So save it before calling compiler code, and restore it afterwards R1 – assumed to always be 0 by the compiler Save and clear it before calling compiler code, restore it after the compiler code returns R2-R31 – used by compiler for various purposes You must follow the compiler’s rules for register usage If you are calling compiler code: Save these before calling; restore after the call returns If you are being called by compiler code: Save first thing in your assembly subroutine; restore right before returning

CS-280 Dr. Mark L. Hornick 14 C and Assembly in Embedded Systems software Entire program in C… …or mix C with assembly. Assembly is generally used for… Critical code (size and efficiency) Accessing certain hardware Existing, proven code already written in assembly

CS-280 Dr. Mark L. Hornick 15 Inline Assembly Useful when writing mostly in C but a few assembly instructions are needed for… Calling assembly subroutines Accessing assembly global variables (Not covered in detail) Maximum efficiency for a small but often used piece of code

CS-280 Dr. Mark L. Hornick 16 Inline assembly – calling a function void main() {... __asm(“rcall sub1"); // Call routine without // a C interface... }

CS-280 Dr. Mark L. Hornick 17 extern variables extern – The definition and label for something… are external (present in another module) will be resolved by the linker extern modifies a declaration Examples Variables – stored in another module Functions – implemented in another module

CS-280 Dr. Mark L. Hornick 18 Accessing an assembly variable from C Assembly.section.data.global counter counter:.word 0x1234 C++ extern int counter; int main() { counter++; }

The volatile keyword When a variable is declared volatile, the compiler will always ensure that an up-to- date value of that variable is manipulated whenever that variable is accesses. CS-280 Dr. Mark L. Hornick 19

Code generated without global variable y declared volatile int fun1() { y=y+3; DDRB = 0xFF; PORTB = 0x00; return y; } CS-280 Dr. Mark L. Hornick be : be: ldsr24, 0x0060 c2: ldsr25, 0x0061 c6:03 96 adiwr24, 0x03; 3 c8: sts0x0061, r25 cc: sts0x0060, r24 d0:2f ef ldir18, 0xFF; 255 d2:27 bb out0x17, r18; 23 d4:18 ba out0x18, r1 ; 24 d6:08 95 ret

Code generated with global variable y declared volatile int fun1() { y=y+3; DDRB = 0xFF; PORTB = 0x00; return y; } CS-280 Dr. Mark L. Hornick be : be: ldsr24, 0x0060 c2: ldsr25, 0x0061 c6:03 96 adiwr24, 0x03; 3 c8: sts0x0061, r25 cc: sts0x0060, r24 d0:2f ef ldir18, 0xFF; 255 d2:27 bb out0x17, r18; 23 d4:18 ba out0x18, r1 ; 24 d6: ldsr24, 0x0060 da: ldsr25, 0x0061 de:08 95 ret