This web page examines bit and bit string 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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Bit manipulation instructions manipulate a specific bit of a bit string (or operand treated as a bit string). Bit clear changes the specified bit to zero. Bit set changes the specified bit to one. Bit change modifies a specified bit, clearing a one bit to zero and setting a zero bit to one. In some processors, the value of the bit before modification is tested. Bit test examines the value of a specified bit.
Bit scan instructions search a bit string for the first bit that is set or cleared (depending on the processor).
BTST Bit Test; Motorola 680x0, Motorola 68300; tests the value of a specified bit in a register or memory location (8 bit memory operands, 32 bit register operands); sets or clears flags
BIT Bit Test; Intel 80x86; tests the value of a specified bit in a general register or memory location; sets or clears flags
BCLR Bit Test and Clear; Motorola 680x0, Motorola 68300; tests the value of a specified bit in a register or memory location (8 bit memory operands, 32 bit register operands), then clears the specified bit to zero; sets or clears flags
BTR Bit Test and Reset; Intel 80x86; tests the value of a specified bit in a general register or memory location, then clears (resets) the specified bit to one; sets or clears flags
BSET Bit Test and Set; Motorola 680x0, Motorola 68300; tests the value of a specified bit in a register or memory location (8 bit memory operands, 32 bit register operands), then sets the specified bit to one; sets or clears flags
BTS Bit Test and Set; Intel 80x86; tests the value of a specified bit in a general register or memory location, then sets the specified bit to one; sets or clears flags
BCHG Bit Test and Change; Motorola 680x0, Motorola 68300; tests the value of a specified bit in a register or memory location (8 bit memory operands, 32 bit register operands), then either clears a one bit to zero or sets a zero bit to one; sets or clears flags
BTC Bit Test and Complement; Intel 80x86; tests the value of a specified bit in a general register or memory location, then complements; sets or clears flags
BSF Bit Scan Forward; Intel 80x86; scans a word (16 bits) or doubleword (32 bits) in memory or a general register from low-order to high-order (starting from bit index zero) for a one-bit and store the index of the first set bit into a register (if no set bit is found, the value of the destination register is undefined); sets or clears flags
BSR Bit Scan Reverse; Intel 80x86; scans a word (16 bits) or doubleword (32 bits) in memory or a general register from high-order to low-order (starting from bit index 15 of a word or index 31 of a doubleword) for a one-bit and store the index of the first set bit into a register (if no set bit is found, the value of the destination register is undefined); sets or clears flags
bit field
Bit field instructions make modifications to bit fields (or operands treated as bit fields). Bit field insert inserts a value into a bit field. Bit field extract extracts a signed or unsigned value from a bit field. Bit field find first one finds the first bit that is set (one) in a bit field. Bit field test evaluates a bit field and sets or clears flags. Bit field test and setevaluates a bit field and set or clear flags then sets the bit field. Bit field test and clearevaluates a bit field and set or clear flags then clears the bit field. Bit field test and changeevaluates a bit field and set or clear flags then changes the bit field.
BFINS Bit Field Insert; Motorola 680x0; inserts a bit field (1 to 32 bits) from a data register to a location in data register or memory located by field offset and field width; sets or clears flags
BFEXTU Bit Field Extract Unsigned; Motorola 680x0; extracts an unsigned bit field (1 to 32 bits) in registers or memory located by field offset and field width and zero extends the field into a data register (32 bits); sets or clears flags
BFEXTS Bit Field Extract Signed; Motorola 680x0; extracts a signed bit field (1 to 32 bits) in registers or memory located by field offset and field width and sign extends the field into a data register (32 bits); sets or clears flags
BFFFO Bit Field Find First One; Motorola 680x0; searches a bit field (1 to 32 bits) in registers or memory located by field offset and field width for the most significant bit that is set to a value of one; set or clears flags
BFTST Bit Field Test; Motorola 680x0; sets condition codes according to the value of a bit field (1 to 32 bits) in registers or memory located by field offset and field width
BIT Bit Test; DEC VAX; performs a logical AND of the bit mask (first operand, register or memory) with the source (second operand, register or memory) without modifying either operand and tests the resulting bits for being all zero (BITB byte, BITW word, BITL longword); sets or clears flags
BFSET Test Bit Field and Set; Motorola 680x0; sets condition codes according to the value of a bit field (1 to 32 bits) in registers or memory located by field offset and field width, then sets the field; sets or clear flags
BIS Bit Set; DEC VAX; performs a logical inclusive OR of the bit mask (first operand, register or memory) with the source (second operand, register or memory), available in two operand (results stored in second operand) and three operand (results stored in third operand) (BISB2 bit set byte 2 operand, BISB3 bit set byte 3 operand, BISW2 bit set word 2 operand, BISW3 bit set word 3 operand, BISL2 bit set longword 2 operand, BISL3 bit set longword 3 operand); sets or clears flags
BFCLR Test Bit Field and Clear; Motorola 680x0; ; sets condition codes according to the value of a bit field (1 to 32 bits) in registers or memory located by field offset and field width, then clears the field; sets or clear flags
BIC Bit Clear; DEC VAX; performs a complimented logical AND of the bit mask (first operand, register or memory) with the source (second operand, register or memory), available in two operand (results stored in second operand) and three operand (results stored in third operand) (BICB2 bit clear byte 2 operand, BICB3 bit clear byte 3 operand, BICW2 bit clear word 2 operand, BICW3 bit clear word 3 operand, BICL2 bit clear longword 2 operand, BICL3 bit clear longword 3 operand); sets or clears flags
BFCHG Test Bit Field and Change; Motorola 680x0; sets condition codes according to the value of a bit field (1 to 32 bits) in registers or memory located by field offset and field width, then complements the field; 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
BSF Bit Scan Forward; Intel 80x86; scans a word (16 bit) or doubleword (32 bit) bit string for the first set (one) bit; scans from low-order to high-order (starting from bit index zero); sets or clears flags
BSR Bit Scan Reverse; Intel 80x86; scans a word (16 bit) or doubleword (32 bit) bit string for the first set (one) bit; scans from high-order to low-order (starting from bit index 15 of a word or index 31 of a doubleword); sets or clears flags
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