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  1. #include <avr/io.h>
  2. #include <stdio.h>
  3.  
  4. #include <util/setbaud.h>
  5.  
  6. void uart_init (void)
  7. {
  8.     UBRR0H = UBRRH_VALUE;
  9.     UBRR0L = UBRRL_VALUE;
  10. #if USE_2X
  11.     UCSR0A |= _BV(U2X0);
  12. #else
  13.     UCSR0A &= ~(_BV(U2X0));
  14. #endif
  15.     UCSR0C = _BV(UCSZ01) | _BV(UCSZ00); /* 8-bit data */ 
  16.     UCSR0B = _BV(RXEN0) | _BV(TXEN0);   /* Enable RX and TX */
  17. }
  18.  
  19. int uart_putchar (char c, FILE *stream)
  20. {
  21.     loop_until_bit_is_set(UCSR0A, UDRE0); /* Wait until data register empty. */
  22.     UDR0 = c;
  23.     return 0;
  24. }
  25.  
  26. FILE uart_output = FDEV_SETUP_STREAM(uart_putchar, NULL, _FDEV_SETUP_WRITE);
  27.  
  28. // Returns the index of the highest 1-bit in an integer, starting with 1,
  29. // or zero if the operand is zero.
  30. // Working version. Uses r16 for temporary and mov's the result into %[res].
  31. #if 0
  32. static uint8_t highest_bit (uint32_t op)
  33. {
  34.     uint8_t res;
  35.     __asm__(
  36.         "    cpse %D[op],__zero_reg__\n"
  37.         "    rjmp foundD%=\n"
  38.         "    cpse %C[op],__zero_reg__\n"
  39.         "    rjmp foundC%=\n"
  40.         "    cpse %B[op],__zero_reg__\n"
  41.         "    rjmp foundB%=\n"
  42.         "    cpse %A[op],__zero_reg__\n"
  43.         "    rjmp foundA%=\n"
  44.         "    clr r16\n"
  45.         "    rjmp end%=\n"
  46.         "foundD%=:\n"
  47.         "    ldi r16,0x18\n"
  48.         "    mov r17,%D[op]\n"
  49.         "    rjmp examine%=\n"
  50.         "foundC%=:\n"
  51.         "    ldi r16,0x10\n"
  52.         "    mov r17,%C[op]\n"
  53.         "    rjmp examine%=\n"
  54.         "foundB%=:\n"
  55.         "    ldi r16,0x08\n"
  56.         "    mov r17,%B[op]\n"
  57.         "    rjmp examine%=\n"
  58.         "foundA%=:\n"
  59.         "    ldi r16,0x00\n"
  60.         "    mov r17,%A[op]\n"
  61.         "examine%=:\n"
  62.         "    bst r17,7\n"
  63.         "    brts bit7%=\n"
  64.         "    bst r17,6\n"
  65.         "    brts bit6%=\n"
  66.         "    bst r17,5\n"
  67.         "    brts bit5%=\n"
  68.         "    bst r17,4\n"
  69.         "    brts bit4%=\n"
  70.         "    bst r17,3\n"
  71.         "    brts bit3%=\n"
  72.         "    bst r17,2\n"
  73.         "    brts bit2%=\n"
  74.         "    bst r17,1\n"
  75.         "    brts bit1%=\n"
  76.         "    subi r16,0xFF\n"
  77.         "    rjmp end%=\n"
  78.         "bit7%=:\n"
  79.         "    subi r16,0xF8\n"
  80.         "    rjmp end%=\n"
  81.         "bit6%=:\n"
  82.         "    subi r16,0xF9\n"
  83.         "    rjmp end%=\n"
  84.         "bit5%=:\n"
  85.         "    subi r16,0xFA\n"
  86.         "    rjmp end%=\n"
  87.         "bit4%=:\n"
  88.         "    subi r16,0xFB\n"
  89.         "    rjmp end%=\n"
  90.         "bit3%=:\n"
  91.         "    subi r16,0xFC\n"
  92.         "    rjmp end%=\n"
  93.         "bit2%=:\n"
  94.         "    subi r16,0xFD\n"
  95.         "    rjmp end%=\n"
  96.         "bit1%=:\n"
  97.         "    subi r16,0xFE\n"
  98.         "end%=:\n"
  99.         "    mov %[res],r16\n"
  100.         : [res] "=r" (res)
  101.         : [op] "r" (op)
  102.         : "r16", "r17"
  103.     );
  104.     return res;
  105. }
  106. #endif
  107.  
  108. // Broken version. Works directly with %[res].
  109. // This code was obtained from the above code like that:
  110. // - remove the final mov instruction,
  111. // - remove r16 from clobber list,
  112. // - replace all use of r16 with %[res].
  113. // It's only "half" broken. While the main loop below should print out the numbers 0-32.
  114. // with this code it prints:
  115. //   0 1 1 1 1 1 1 1 1 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
  116. static uint8_t highest_bit (uint32_t op)
  117. {
  118.     uint8_t res;
  119.     __asm__(
  120.         "    cpse %D[op],__zero_reg__\n"
  121.         "    rjmp foundD%=\n"
  122.         "    cpse %C[op],__zero_reg__\n"
  123.         "    rjmp foundC%=\n"
  124.         "    cpse %B[op],__zero_reg__\n"
  125.         "    rjmp foundB%=\n"
  126.         "    cpse %A[op],__zero_reg__\n"
  127.         "    rjmp foundA%=\n"
  128.         "    clr %[res]\n"
  129.         "    rjmp end%=\n"
  130.         "foundD%=:\n"
  131.         "    ldi %[res],0x18\n"
  132.         "    mov r17,%D[op]\n"
  133.         "    rjmp examine%=\n"
  134.         "foundC%=:\n"
  135.         "    ldi %[res],0x10\n"
  136.         "    mov r17,%C[op]\n"
  137.         "    rjmp examine%=\n"
  138.         "foundB%=:\n"
  139.         "    ldi %[res],0x08\n"
  140.         "    mov r17,%B[op]\n"
  141.         "    rjmp examine%=\n"
  142.         "foundA%=:\n"
  143.         "    ldi %[res],0x00\n"
  144.         "    mov r17,%A[op]\n"
  145.         "examine%=:\n"
  146.         "    bst r17,7\n"
  147.         "    brts bit7%=\n"
  148.         "    bst r17,6\n"
  149.         "    brts bit6%=\n"
  150.         "    bst r17,5\n"
  151.         "    brts bit5%=\n"
  152.         "    bst r17,4\n"
  153.         "    brts bit4%=\n"
  154.         "    bst r17,3\n"
  155.         "    brts bit3%=\n"
  156.         "    bst r17,2\n"
  157.         "    brts bit2%=\n"
  158.         "    bst r17,1\n"
  159.         "    brts bit1%=\n"
  160.         "    subi %[res],0xFF\n"
  161.         "    rjmp end%=\n"
  162.         "bit7%=:\n"
  163.         "    subi %[res],0xF8\n"
  164.         "    rjmp end%=\n"
  165.         "bit6%=:\n"
  166.         "    subi %[res],0xF9\n"
  167.         "    rjmp end%=\n"
  168.         "bit5%=:\n"
  169.         "    subi %[res],0xFA\n"
  170.         "    rjmp end%=\n"
  171.         "bit4%=:\n"
  172.         "    subi %[res],0xFB\n"
  173.         "    rjmp end%=\n"
  174.         "bit3%=:\n"
  175.         "    subi %[res],0xFC\n"
  176.         "    rjmp end%=\n"
  177.         "bit2%=:\n"
  178.         "    subi %[res],0xFD\n"
  179.         "    rjmp end%=\n"
  180.         "bit1%=:\n"
  181.         "    subi %[res],0xFE\n"
  182.         "end%=:\n"
  183.         : [res] "=r" (res)
  184.         : [op] "r" (op)
  185.         : "r17"
  186.     );
  187.     return res;
  188. }
  189.  
  190. int main ()
  191. {
  192.     uart_init();
  193.     stdout = &uart_output;
  194.  
  195.     printf("%d", highest_bit(0));
  196.     for (int i = 0; i < 32; i++) {
  197.         printf(" %d", highest_bit((uint32_t)1 << i));
  198.     }
  199.     printf("\n");
  200. }
  201.  
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https://ideone.com/JTpYma
language:
C99 (gcc 8.3)
created:
10 years ago
visibility:
public

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