November 2, 2019

A quick look at the AVR architecture

I had the final build of the blink example laying around. So I ran the following command: 
avr-objdump -d main.elf > main.dump
This yielded a far more readable result than using the -S switch to the compiler and examining the resulting main.s file. 
00000000 <__ctors_end>:
   0:   09 c0           rjmp    .+18            ; 0x14 <__init>
   2:   0e c0           rjmp    .+28            ; 0x20 <__bad_interrupt>
   4:   0d c0           rjmp    .+26            ; 0x20 <__bad_interrupt>
   6:   0c c0           rjmp    .+24            ; 0x20 <__bad_interrupt>
   8:   0b c0           rjmp    .+22            ; 0x20 <__bad_interrupt>
   a:   0a c0           rjmp    .+20            ; 0x20 <__bad_interrupt>
   c:   09 c0           rjmp    .+18            ; 0x20 <__bad_interrupt>
   e:   08 c0           rjmp    .+16            ; 0x20 <__bad_interrupt>
  10:   07 c0           rjmp    .+14            ; 0x20 <__bad_interrupt>
  12:   06 c0           rjmp    .+12            ; 0x20 <__bad_interrupt>

00000014 <__init>:
  14:   11 24           eor     r1, r1
  16:   1f be           out     0x3f, r1        ; 63
  18:   cf e9           ldi     r28, 0x9F       ; 159
  1a:   cd bf           out     0x3d, r28       ; 61
  1c:   02 d0           rcall   .+4             ; 0x22 

  1e:   17 c0           rjmp    .+46            ; 0x4e <_exit>

00000020 <__bad_interrupt>:
  20:   ef cf           rjmp    .-34            ; 0x0 <__ctors_end>

00000022 
:
  22:   81 e0           ldi     r24, 0x01       ; 1
  24:   87 bb           out     0x17, r24       ; 23
  26:   91 e0           ldi     r25, 0x01       ; 1
  28:   80 e0           ldi     r24, 0x00       ; 0
  2a:   98 bb           out     0x18, r25       ; 24
  2c:   2f e3           ldi     r18, 0x3F       ; 63
  2e:   35 ef           ldi     r19, 0xF5       ; 245
  30:   46 e0           ldi     r20, 0x06       ; 6
  32:   21 50           subi    r18, 0x01       ; 1
  34:   30 40           sbci    r19, 0x00       ; 0
  36:   40 40           sbci    r20, 0x00       ; 0
  38:   e1 f7           brne    .-8             ; 0x32 
  3a:   00 c0           rjmp    .+0             ; 0x3c 
  3c:   00 00           nop
  3e:   88 bb           out     0x18, r24       ; 24
  40:   ef e2           ldi     r30, 0x2F       ; 47
  42:   f5 e7           ldi     r31, 0x75       ; 117
  44:   31 97           sbiw    r30, 0x01       ; 1
  46:   f1 f7           brne    .-4             ; 0x44 <__SREG__+0x5>
  48:   00 c0           rjmp    .+0             ; 0x4a <__SREG__+0xb>
  4a:   00 00           nop
  4c:   ee cf           rjmp    .-36            ; 0x2a 

0000004e <_exit>:
  4e:   f8 94           cli

00000050 <__stop_program>:
  50:   ff cf           rjmp    .-2             ; 0x50 <__stop_program>
Even without knowing anything about AVR assembly this yields a pretty readable listing.

Note that there is a convention of R1 being used as a "zero register". This is entirely a convention, but note that the first thing the "__init" routine does is to clear it (and then use it). Code generated by avr_gcc indeed depends on this and if interrupts are used and r1 is not maintained there can be trouble. The fact that r1 is a zero register could be used to better advantage in this code, but there are limits to what gcc can do.

Have any comments? Questions? Drop me a line!

Tom's Electronics pages / tom@mmto.org