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  1. Lsl/Lsr/Asl/Rol/Ror

    Affected Flags
    C V N Z
    X
    X X

    lsl : Logical shift left
    lsr : Logical shift right
    asr : Arithmetic Shift Right
    rol : Circular rotation left
    ror : Circulat rotation right

    These instructions are doing different kind of bit shifting. Instructions terminated by "L" are shifting to left while instructions terminated by "R" are shifting to right. There are 3 kind of bit shiting. Lsl and Lsr are doing simple shifting for unsigned numbers. The Rol and Ror are circular bit shift with the carry. Asr is doing the same thing than Lsr but for signed number : the sign is preserved. There is no Asl instruction as Lsl is working also for signed number. An overflow condition with signed number and Lsl can be detected with the Bso conditionnal jump instruction.
    Bits being thrown out by shift instructions are put in the C Flag from the status register.
    In many case, shift instructions is a fast way to make multiplication and division by a power of two numbers.


    Here are some graphics to explain behavior of these instructions :

    Shift instruction forms and variants

    Forms and variants

    rx = shifted register.
    ry = Register containing the number of shifts.

    Form Effect
    lsl.b ry,rx
    Logical Shift Left
    lsl.w ry,rx
    lsl.l ry,rx
    lsr.b ry,rx Logical Shift Right
    lsr.w ry,rx
    lsr.l ry,rx
    asr.b ry,rx
    Arithmetic Shift Right
    asr.w ry,rx
    asr.l ry,rx
    rol.b ry,rx Circular Shift Left
    rol.w ry,rx
    rol.l ry,rx
    ror.b ry,rx Circular Shift Right
    ror.w ry,rx
    ror.l ry,rx

    Instruction encoding

    S = Number of the register containing shift count
    D = Shifted register number
    Instruction Op
    Code
    Cycles Encoding
    lsl.b ry,rx 14 6 00010100 SSSSDDDD
    lsl.w ry,rx 54 6 01010100 SSSSDDDD
    lsl.l ry,rx 94 6 10010100 SSSSDDDD
    lsr.b ry,rx 15 6 00010101 SSSSDDDD
    lsr.w ry,rx 55 6 01010101 SSSSDDDD
    lsr.l ry,rx 95 6 10010101 SSSSDDDD
    asr.b ry,rx 13 6 00010011 SSSSDDDD
    asr.w ry,rx 53 6 01010011 SSSSDDDD
    asr.l ry,rx 93 6 10010011 SSSSDDDD
    rol.b ry,rx 16 6 00010110 SSSSDDDD
    rol.w ry,rx 56 6 01010110 SSSSDDDD
    rol.l ry,rx 96 6 10010110 SSSSDDDD
    ror.b ry,rx 17 6 00010111 SSSSDDDD
    ror.w ry,rx 57 6 01010111 SSSSDDDD
    ror.l ry,rx 97 6 10010111 SSSSDDDD

    Code Examples

    Note: This example is for understanding and can be optimized.

    // Example : 8x8 bit graphic 90° rotation

    ; r0 = pointer to source array of 8 bytes
    ; r1 = pointer to destination array of 8 bytes
    rotate: pushregs r2-r3/r7-r14 move.b (r0),r7 inclr move.b (r0),r8 inclr move.b (r0),r9 inclr move.b (r0),r10 inclr move.b (r0),r11 inclr move.b (r0),r12 inclr move.b (r0),r13 inclr move.b (r0),r14 movex.n #1,r0 movex.n #8,r3 loop: lsl.b r0, r7 rol.b r0, r2 lsl.b r0, r8 rol.b r0, r2 lsl.b r0, r9 rol.b r0, r2 lsl.b r0, r10 rol.b r0, r2 lsl.b r0, r11 rol.b r0, r2 lsl.b r0, r12 rol.b r0, r2 lsl.b r0, r13 rol.b r0, r2 lsl.b r0, r14 rol.b r0, r2 move.b r2,(r1) inclr dec.l #1,r3 bne loop popregs r2-r3/r7-r14 rts
     
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