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

Lsl/Lsr/Asl/Rol/Ror

Home