Cornell University
Electrical and Computer Engineering 4760
AVR mega644/1284
Mixing assembler with GCC


It is possible to mix assembler and GCC in different ways:

Before you actually write any assembly code you will need to read the instruction set architecture and description of AVR opcodes, and look at a bunch of assembler examples. Some examples are below. There are some tutorials, for instance scienceprog and Mixing C and asm. I find that the best way to learn assembler is to look at the assembler output of the compiler. In AVRstudio projects, the *.lss assembler listing file is in the default folder (in the project folder). Code up a few lines of C, open the lss file, search for a line of C (included as comment by the compiler), and see what the compiler did. There is an example below of compiler output from a video generator I wrote in C (assembler comments added for this page). The code line is in a function with x the first (char) parameter and y the second.

;C comment: int i = (x >> 3) + (int)y * bytes_per_line ;
ldi	r24, 0x14	; bytes_per_line=20=0x14
mul	r22, r24 	; since y was the second (char) parameter of a function call, it is in r22
movw	r26, r0		; 2-byte move to get product at r27:r26
eor	r1, r1		; MUST clear r1 after a mult 	
mov	r24, r30	; The compiler had moved the first parameter from r24 to r30	
lsr	r24		; 3x lsr for the >>3 
lsr	r24
lsr	r24
add	r26, r24	; do the add
adc	r27, r1 	; and add the carry to the high byte using r1=0 register

The lss file can tell you other stuff also. If you search for __vectors, you will get the interrupt service routine entry points. You will see by following the undefined interrupt vectors, that GCC defaults to resetting the MCU for any undefined interrupt. The zero entry point is the RESET vector where program execution starts. Searching for that address will lead you to the MCU and C initialization code. The first few lines of the reset code are shown below with my comments added:

 eor	r1, r1 		; clear r1 (C assumes r1 equals zero)
 out	0x3f, r1	; zero the SREG which is i/o register 0x3f
 ldi	r28, 0xFF	; load the low byte of the top-of-memory address
 ldi	r29, 0x40	; load the top byte of the top-of-memory address
 out	0x3e, r29	; store the top byte in top byte of stack pointer (i/o register 0x3e)
 out	0x3d, r28	; store the low byte in low byte of stack pointer (i/o register 0x3d)

The next few lines shown in the lss file clear memory and set up the C environment, then jump to main. The map file in the same folder will show you where the variables are stored in RAM.

Syntax and registers

Global variables defined in C are available to the assembler. For a global variable defined in C as volatile char vname;
Using the declared C variable name in a load/store command like those below loads/stores the value of the variable into the register.
lds r18,vname
sts vname, r18

Integer variables declared in C as int vname; may be loaded using:
lds r18, vname ; lower byte
lds r19, vname+1 ; high byte

An array can be indexed by loading the base address into a register pair r27:r26 (called the X register), or r31:r30 (called the Z register) and adding the index. In the following case, the max index was less than 256, so the addition was extended to 16-bits by adding zero (with carry) to the high byte of the pointer.
The C declaration is volatile unsigned char samples[255].
ldi r30, lo8(samples) ; use ldi for a pointer
ldi r31, hi8(samples)
lds r18, index
add r30, r18
adc r31, r1 ; compiler enforces r1 = 0 ; curent array point now in Z

Doing a ld r18,Z loads the data value from the stored array. Using ld r18,Z+ autoincrements the address after loading.

Parameters can be passed to functions and returned. The first parameter is passed in r25:r24, the second in r23:r22 down to r8. All arguments are aligned to start in even-numbered registers (odd-sized arguments, including char, have one free register above them). Return values: 8-bit in r24 (not r25!), 16-bit in r25:r24, up to 32 bits in r22-r25, up to 64 bits in r18-r25. The following example is from the fixed point project descipbed in the Examples section below. The first bit of code is from the calling program where you can see how the input and output parameters are loaded/stored.

; prod = multfix(fix1, fix2) ;
lds	r22, 0x0252 	; get low byte fix2
lds	r23, 0x0253 	; get high byte fix2
lds	r24, 0x024C 	; get low byte fix1
lds	r25, 0x024D 	; get high byte fix1
call	0x19cc	; 0x19cc  call the external mult routine
sts	0x024B, r25 	; store high byte prod
sts	0x024A, r24 	; store low byte prod

The second bit of code is the multfix routine called by the above assembler code. It is just the contents of the multfix.S file with the global symbol resolved into an address. The code shows how the parameters are used to form the fixed point product by multiplying all 4 pairs of high and low bytes.
000019cc <multfix>: ; the resolved global symbol
;input parameters are in r23:r22(hi:lo) and r25:r24 ;b aready in right place -- 2nd parameter is in r22:23 ;load a -- first parameter is in r24:25 need to move it to make room for output movw r20, r24 ; open up result return registers (notice movw is moveword) muls r23, r21 ; (signed)ahigh * (signed)bhigh mov r25, r0 ; only need low byte of high multiply mul r22, r20 ; alow * blow mov r24, r1 ; only need high byte of low mult mulsu r23, r20 ; (signed)ahi * blo add r24, r0 ; add product to result adc r25, r1 mulsu r21, r22 ; (signed)bhi * alo add r24, r0 ; add product to result adc r25, r1 clr r1 ; required by GCC ;return values are in 25:r24 (hi:lo) ret

Registers r18-r27 and r30-r31 can be used in a function without saving. The compiler saves r18-r27 and r30-r31 when you enter a function, so these can be used any way you want. Registers r2-r17 must be saved by you. In inline code with register contraints, the compiler will attempt to optimize your register use, but the directives are bewildering. See the fixed-point multiply for an example macro. If you use inline assembler code without constraints, you must save registers that you use. Register r0 is considered a temporary register. It will be changed by any C code (except interrupt handlers which save it), and may be used to store a byte within one piece of assembler code. Register r1 is assumed to be always zero in any C code. It may be used to store a byte within one piece of assembler code, but must then be cleared after use (clr r1). This includes any use of the multiply [f]mul[s[u]] instructions, which return their result in r1:r0. Interrupt handlers save and clear r1 on entry, and restore r1 on exit (in case it was non-zero). This paragraph taken largely from the nongnu docs.

Registers r2 to r7 can be locked to global variable names. The syntax to bind variable var_name to register 3 is:
register unsigned char var_name asm("r3");
Be very careful using this feature. It is easy to generate register conflicts and you may restrict the optimization that the compiler can perform. See also nongnu docs.


See also

Copyright Cornell University August 16, 2012