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Re: [avr-gcc-list] GCC Functions

From: Joerg Wunsch
Subject: Re: [avr-gcc-list] GCC Functions
Date: Wed, 28 Nov 2001 16:19:54 +0100
User-agent: Mutt/1.2.5i

As Patrick Lanphier wrote:

> Well the lights do not turn on when using the function when they do with
> the simple assignment statement.

They would even turn on without the assignment statement. ;-)

Again, the generated code looks OK.  If this is actually your entire
program, something in your AVR must be broken.  The only other option
is that something is wrong with the stack (in which case the function
call would never return), but normally, the framework of avr-gcc
handles the stack initialization etc. automatically.  (If you look
into the generated assembler code, you can easily spot this.)

You can easily verify the result by running "avr-objdump --disassemble

Disassembly of section .text:

00000000 <.__start_of_init__>:
   0:   0a c0           rjmp    .+20            ; 0x16
0000004a <led1>:
  4a:   cf 93           push    r28
  4c:   df 93           push    r29
  4e:   cd b7           in      r28, 0x3d       ; 61
  50:   de b7           in      r29, 0x3e       ; 62
  52:   10 92 60 00     sts     0x0060, r1
  56:   df 91           pop     r29
  58:   cf 91           pop     r28
  5a:   08 95           ret

Register r1 contains always the constant 0 in avr-gcc.
The variable at 0x60 is "ledlight".
Note that all the fiddling with the frame pointer in r28/r29
is moot; if you use -O2, avr-gcc omits it.

0000005c <main>:
  5c:   cf ed           ldi     r28, 0xDF       ; 223
  5e:   d0 e0           ldi     r29, 0x00       ; 0
  60:   de bf           out     0x3e, r29       ; 62
  62:   cd bf           out     0x3d, r28       ; 61

This is the initialization of the stack pointer, pointing to the top
of RAM.  avr-gcc always generates code for a 16-bit stack pointer,
although the '2313 only uses the lower stack pointer (register 0x3d).
Obviously :), there should be no program variables located at the top
of RAM.

  64:   8f ef           ldi     r24, 0xFF       ; 255
  66:   87 bb           out     0x17, r24       ; 23
  68:   8f ef           ldi     r24, 0xFF       ; 255
  6a:   88 bb           out     0x18, r24       ; 24

These are the outputs to DDRB and PORTB.

  6c:   ee df           rcall   .-36            ; 0x4a

The call to led1().

  6e:   10 92 60 00     sts     0x0060, r1

The same as inline code inside main().

  72:   80 91 60 00     lds     r24, 0x0060
  76:   88 bb           out     0x18, r24       ; 24
  78:   fc cf           rjmp    .-8             ; 0x72

Your loop.

J"org Wunsch                                           Unix support engineer
address@hidden        http://www.interface-systems.de/~j/

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