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  1. uint8_t my_log2 (uint32_t op)
  2. {
  3.     uint8_t res;
  4.     asm(
  5.         "    cpse %D[op],__zero_reg__\n"
  6.         "    rjmp foundD%=\n"
  7.         "    cpse %C[op],__zero_reg__\n"
  8.         "    rjmp foundC%=\n"
  9.         "    cpse %B[op],__zero_reg__\n"
  10.         "    rjmp foundB%=\n"
  11.         "    cpse %A[op],__zero_reg__\n"
  12.         "    rjmp foundA%=\n"
  13.         "    clr r16\n"
  14.         "    rjmp end%=\n"
  15.         "foundD%=:\n"
  16.         "    ldi r16,0x18\n"
  17.         "    mov r17,%D[op]\n"
  18.         "    rjmp examine%=\n"
  19.         "foundC%=:\n"
  20.         "    ldi r16,0x10\n"
  21.         "    mov r17,%C[op]\n"
  22.         "    rjmp examine%=\n"
  23.         "foundB%=:\n"
  24.         "    ldi r16,0x08\n"
  25.         "    mov r17,%B[op]\n"
  26.         "    rjmp examine%=\n"
  27.         "foundA%=:\n"
  28.         "    ldi r16,0x00\n"
  29.         "    mov r17,%A[op]\n"
  30.         "examine%=:\n"
  31.         "    bst r17,7\n"
  32.         "    brts bit7%=\n"
  33.         "    bst r17,6\n"
  34.         "    brts bit6%=\n"
  35.         "    bst r17,5\n"
  36.         "    brts bit5%=\n"
  37.         "    bst r17,4\n"
  38.         "    brts bit4%=\n"
  39.         "    bst r17,3\n"
  40.         "    brts bit3%=\n"
  41.         "    bst r17,2\n"
  42.         "    brts bit2%=\n"
  43.         "    bst r17,1\n"
  44.         "    brts bit1%=\n"
  45.         "    subi r16,0xFF\n"
  46.         "    rjmp end%=\n"
  47.         "bit7%=:\n"
  48.         "    subi r16,0xF8\n"
  49.         "    rjmp end%=\n"
  50.         "bit6%=:\n"
  51.         "    subi r16,0xF9\n"
  52.         "    rjmp end%=\n"
  53.         "bit5%=:\n"
  54.         "    subi r16,0xFA\n"
  55.         "    rjmp end%=\n"
  56.         "bit4%=:\n"
  57.         "    subi r16,0xFB\n"
  58.         "    rjmp end%=\n"
  59.         "bit3%=:\n"
  60.         "    subi r16,0xFC\n"
  61.         "    rjmp end%=\n"
  62.         "bit2%=:\n"
  63.         "    subi r16,0xFD\n"
  64.         "    rjmp end%=\n"
  65.         "bit1%=:\n"
  66.         "    subi r16,0xFE\n"
  67.         "end%=:\n"
  68.         "    mov %[res],r16\n"
  69.         : [res] "=r" (res)
  70.         : [op] "r" (op)
  71.         : "r16", "r17"
  72.     );
  73.     return res;
  74. }
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https://ideone.com/8Gtk4q
language:
C99 (gcc 8.3)
created:
10 years ago
visibility:
public

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