[ENGN3213 Home]

gcc Assembler Syntax and Directives

The gcc assembler syntax is a bit more complicated than the syntax used by 68kasm. Please refer to the documentation (at end of this lab manual and on ENGN3213 website). We will become familiar with the basics by working with examples and studying the assembly code produced by the gcc C compiler.

Download the file clab70s.s:

/*    clab70s.s   - m68k-coff-as version of clab52s.s  */

      .text
      .even
      .global  main
main:                        /* start of main program */
      clr.w    sum           /* clear variable sum */
      move.w   count,%d0     /* load count value */
loop: add.w    %d0,sum       /* add d0 to sum */
      sub.w    #1,%d0        /* decrement counter */
      bne      loop          /* loop if counter not zero */
      move.w   sum,%d7
      rts                    /* return to caller */     
sum:    .space 2
count:  .word 25

This is a version of clab52s.s ment for the gcc assembler, m68k-coff-as. Note the following points:

• The syntax is closely related to C syntax.

• The program begins with a label called main, and this label is global so that it is visible to the linker m68k-coff-ld.

• main will be called by the startup routine which will be linked in. The startup routine crt0.o sets up reset values for PC, SP, etc.

• Registers must always have a percent (%) in front.

• Note how variables and constants are set up.

• At completion, the program returns control to the startup routine using rts.

[ENGN3213 Home]