SHL shifts the bits of the destination operand upward (toward the most significant bit) by the number of bit positions specified in the second operand (count). As bits are transferred out of the left (high-order) end of the destination, zero bits are shifted into the right (low-order) end. The carry flag (CF) is set equal to the last bit shifted out of the left end.
The shift is repeated the number of times indicated by the second operand, which is either an immediate 8-bit value (max. 1 on the 8086 processor) or the contents of the CL register. To reduce the maximum execution time, the 80186+ uses only the lower 5 bits of the count, limiting the count value to 31; the 8086 uses all 8 bits of count.
If the count operand is not an immediate 1, the overflow flag (OF) is undefined; otherwise SHL sets OF to 0 if destination's sign bit was not changed by the operation, to 1 if the sign bit was changed.
SHL multiplies an unsigned integer by a power-of-two.
Example: shl ax, 1 ; Shift left 1 bit rcl dx, 1 ; Propagate carry
Note SHL is the same instruction as SAL (shift arithmetic left).