Assembly Language

This web page examines shift and rotate instructions in assembly language. Specific examples of instructions from various processors are used to illustrate the general nature of assembly language.

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• table of contents for assembly language section
• shift and rotate
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shift and rotate

Shift and rotate instructions move bit strings (or operand treated as a bit string).

Shift instructions move a bit string (or operand treated as a bit string) to the right or left, with excess bits discarded (although one or more bits might be preserved in flags). In arithmetic shift left or logical shift left zeros are shifted into the low-order bit. In arithmetic shift right the sign bit (most significant bit) is shifted into the high-order bit. In logical shift right zeros are shifted into the high-order bit.

Rotate instructions are similar to shift instructions, ecept that rotate instructions are circular, with the bits shifted out one end returning on the other end. Rotates can be to the left or right. Rotates can also employ an extend bit for multi-precision rotates.

A swap instruction swaps the high and low order portions of a register or contents of a series of memory locations.

The carry bit typically receives the last bit shifted out of the operand. Sometimes an extend bit will receive the last bit shifted out also. Somtimes an overflow bit is used to indicate a sign change has occurred.

• ASH Arithmetic Shift; DEC VAX; performs a bit shift on a longword or quadword, the first operand is a byte count, the second operand is the source longword or quadword in registers or memory, the third operand is the destination longword or quadword in registers or memory, positive counts causes a left shift with zeros entering in the least significant bits, negative count causes a right shift with the most significant bit being copied into the most significant bit, a zero count results in the destination being replaced by the unmodified source (ASHL arithmetic shift longword, ASHQ arithmetic shift quadword); sets or clears flags
• ASL Arithmetic Shift Left; Motorola 680x0, Motorola 68300; shifts the contents of a data register (8, 16, or 32 bits) or memory location (16 bits) to the left (towards most significant bit) by a specified amount (by 1 to 8 bits for an immediate operation on a data register, by the contents of a data register modulo 64 for a data register, or by 1 bit only for a memory location), with the high-order bit being shifted into the carry and extend flags, zeros shifted into the low-order bit, overflow flag indicating a change of sign; sets or clear flags
• SAL Shift Arithmetic Left; Intel 80x86; shifts the contents of a data register or memory location (8, 16, or 32 bits) to the left (towards most significant bit) by a specified amount (by 1, by 0 to 31 bits specified by an immediate operand, or by 0-31 bits specified by the contents of the CL register), with the high-order bit being shifted into the carry flag, zeros shifted into the low-order bit; sets or clear flags
• ASR Arithmetic Shift Right; Motorola 680x0, Motorola 68300; shifts the contents of a data register (8, 16, or 32 bits) or memory location (16 bits) to the right (towards the least significant bit) by a specified amount (by 1 to 8 bits for an immediate operation on a data register, by the contents of a data register modulo 64 for a data register, or by 1 bit only for a memory location), with the low-order bit being shifted into the carry and extend flags, the original high-order bit being replicated and shifted into the high-order bit; sets or clear flags
• SAR Shift Arithmetic Right; Intel 80x86; shifts the contents of a data register or memory location (8, 16, or 32 bits) to the right (towards least significant bit) by a specified amount (by 1, by 0 to 31 bits specified by an immediate operand, or by 0-31 bits specified by the contents of the CL register), with the low-order bit being shifted into the carry flag, the original high-order bit being replicated and shifted into the high-order bit; sets or clear flags
• SLA Shift Left A-register; MIX; byte shift the contents of the A-register (leaving sign unchanged) to the left by the designated number of bytes, with zeros shifted in to low order bytes
• SLAX Shift Left AX-register; MIX; byte shift the contents of the A-register and X-register pair (leaving signs unchanged) to the left by the designated number of bytes, with zeros shifted in to low order bytes
• LSL Logical Shift Left; Motorola 680x0, Motorola 68300; shifts the contents of a data register (8, 16, or 32 bits) or memory location (16 bits) to the left (towards most significant bit) by a specified amount (by 1 to 8 bits for an immediate operation on a data register, by the contents of a data register modulo 64 for a data register, or by 1 bit only for a memory location), with the high-order bit being shifted into the carry and extend flags, zeros shifted into the low-order bit; sets or clear flags
• SHL Shift Logical Left; Intel 80x86; shifts the contents of a data register or memory location (8, 16, or 32 bits) to the left (towards most significant bit) by a specified amount (by 1, by 0 to 31 bits specified by an immediate operand, or by 0-31 bits specified by the contents of the CL register), with the high-order bit being shifted into the carry flag, zeros shifted into the low-order bit; sets or clear flags
• SRA Shift Right A-register; MIX; byte shift the contents of the A-register (leaving sign unchanged) to the right by the designated number of bytes, with zeros shifted in to high order bytes
• SRAX Shift Right AX-register; MIX; byte shift the contents of the A-register and X-register pair (leaving signs unchanged) to the right by the designated number of bytes, with zeros shifted in to high order bytes
• LSR Logical Shift Right; Motorola 680x0, Motorola 68300; shifts the contents of a data register (8, 16, or 32 bits) or memory location (16 bits) to the right (towards the least significant bit) by a specified amount (by 1 to 8 bits for an immediate operation on a data register, by the contents of a data register modulo 64 for a data register, or by 1 bit only for a memory location), with the low-order bit being shifted into the carry and extend flags, and zeros shifted into the high-order bit; sets or clear flags
• SHR Shift Logical Right; Intel 80x86; shifts the contents of a data register or memory location (8, 16, or 32 bits) to the right (towards least significant bit) by a specified amount (by 1, by 0 to 31 bits specified by an immediate operand, or by 0-31 bits specified by the contents of the CL register), with the low-order bit being shifted into the carry flag, and zeros shifted into the high-order bit; sets or clear flags
• SHLD Double Precision Shift Left; Intel 80x86; shifts the contents of a general purpose register (16 or 32 bits) to the left (towards most significant bit) by a specified amount (by 0 to 31 bits specified by an immediate operand or by 0-31 bits specified by the contents of the CL register) with the high-order bits being shifted into a general purpose register or memory location (the source register is unchanged); used to implement multiprecision shifts, “bit blts” (BIT BLock Transfers), or bit string extracts and inserts; sets or clear flags
• SHRD Double Precision Shift Right; Intel 80x86; shifts the contents of a general purpose register (16 or 32 bits) to the right (towards least significant bit) by a specified amount (by 0 to 31 bits specified by an immediate operand or by 0-31 bits specified by the contents of the CL register) with the low-order bits being shifted into a general purpose register or memory location (the source register is unchanged); used to implement multiprecision shifts, “bit blts” (BIT BLock Transfers), or bit string extracts and inserts; sets or clear flags
• ROTL Rotate Long; DEC VAX; performs a bit rotate on a longword, the first operand is a byte count, the second operand is the source longword in registers or memory, the third operand is the destination longword in registers or memory, positive counts causes a left rotate, negative count causes a right rotate, a zero count results in the destination being replaced by the unmodified source, bits shifted out one end are rotated back in the other end; sets or clears flags
• ROL Rotate Left; Motorola 680x0, Motorola 68300; rotates the contents of a data register (8, 16, or 32 bits) or a memory location (16 bits) to the left (towards the most significant bit) by a specified amount (by 1 to 8 bits for an immediate operation on a data register, by the contents of a data register modulo 64 for a data register, or by 1 bit only for a memory location), with the high-order bit rotating into both the carry flag and the low-order bit; sets or clear flags
• ROL Rotate Left; Intel 80x86; rotates the contents of a general purpose register or a memory location (8, 16, or 32 bits) to the left (towards the most significant bit) by a specified amount (by 1 bit or by 0 to 31 bits specified by an immediate operand or by the contents of the CL register); sets or clear flags
• SLC Shift Left AX-register Circularly; MIX; byte shift the contents of the A-register and X-register pair (leaving signs unchanged) to the left by the designated number of bytes, with bytes shifted off low order end entering on high order end
• ROR Rotate Right; Motorola 680x0, Motorola 68300; rotates the contents of a data register (8, 16, or 32 bits) or a memory location (16 bits) to the right (towards the least significant bit) by a specified amount (by 1 to 8 bits for an immediate operation on a data register, by the contents of a data register modulo 64 for a data register, or by 1 bit only for a memory location), with the low-order bit rotating into both the carry flag and the high-order bit; sets or clear flags
• ROR Rotate Right; Intel 80x86; rotates the contents of a general purpose register or a memory location (8, 16, or 32 bits) to the right (towards the least significant bit) by a specified amount (by 1 bit or by 0 to 31 bits specified by an immediate operand or by the contents of the CL register); sets or clear flags
• SRC Shift Right AX-register Circularly; MIX; byte shift the contents of the A-register and X-register pair (leaving signs unchanged) to the right by the designated number of bytes, with bytes shifted off high order end entering on low order end
• ROXL Rotate Left with Extend; Motorola 680x0, Motorola 68300; rotates the contents of a data register (8, 16, or 32 bits) or a memory location (16 bits) to the left (towards the most significant bit) through the extend bit by a specified amount (by 1 to 8 bits for an immediate operation on a data register, by the contents of a data register modulo 64 for a data register, or by 1 bit only for a memory location), with the high-order bit rotating into both the carry flag and the extend bit and the extend bit rotating into the low-order bit; sets or clear flags
• RCL Rotate Through Carry Left; Intel 80x86; rotates the contents of a general purpose register or a memory location (8, 16, or 32 bits) to the left (towards the most significant bit) by a specified amount (by 1 bit or by 0 to 31 bits specified by an immediate operand or by the contents of the CL register) with the carry flag (CF) being treated as a high-order one-bit extension of the destination operand; sets or clear flags
• ROXR Rotate Right with Extend; Motorola 680x0, Motorola 68300; rotates the contents of a data register (8, 16, or 32 bits) or a memory location (16 bits) to the right (towards the least significant bit) through the extend bit by a specified amount (by 1 to 8 bits for an immediate operation on a data register, by the contents of a data register modulo 64 for a data register, or by 1 bit only for a memory location), with the low-order bit rotating into both the carry flag and the extend bit and the extend bit rotating into the high-order bit; sets or clear flags
• RCR Rotate Through Carry Right; Intel 80x86; rotates the contents of a general purpose register or a memory location (8, 16, or 32 bits) to the right (towards the least significant bit) by a specified amount (by 1 bit or by 0 to 31 bits specified by an immediate operand or by the contents of the CL register) with the carry flag (CF) being treated as as a low-order one-bit extension of the destination operand; sets or clear flags
• SWAP Swap; Motorola 680x0, Motorola 68300; exchanges the high order word (16 bits) with the low order word (16 bits) of a data register; sets or clears flags

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