This web page examines logical 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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Logical instructions typically work on a bit by bit basis, although some processors use the entire contents of the operands as whole flags (zero or not zero input, zero or negative one output). Typical logical operations include logical negation or logical complement (NOT), logical and (AND), logical inclusive or (OR or IOR), and logical exclusive or (XOR or EOR). Logical tests are a comparison of a value to a bit string (or operand treated as a bit string) of all zeros. Some processors have an instruction that sets or clears a bit or byte in registers or memory based on the processor condition codes.
NOT Logical Complement; Motorola 680x0, Motorola 68300; calculates the one’s complement (logical negation) of the contents of memory or a register (8, 16, or 32 bits); sets or clears flags
NOT One's Complement Negation; Intel 80x86; calculates the one’s complement (logical negation) of the contents of memory or a register (8, 16, or 32 bits); does not modify flags
AND And Logical; Motorola 680x0, Motorola 68300; performs a logical AND of a source operand with a destination operand and stores the result in the destination operand (8, 16, or 32 bits), one of the two operands must be a data register, the other operand can be the contents of any register or memory location; sets or clears flags
ANDI And Immediate; Motorola 680x0, Motorola 68300; performs a logical AND of the contents of a register or memory location (8, 16, or 32 bits) with an immediate; sets or clears flags
AND Logical AND; Intel 80x86; performs a logical AND between two registers, a register and contents of a memory location, or an immediate operand and either the contents of a register or the contents of a memory location; byte (8 bits), word (16 bits), or doubleword (32 bits); sets or clears flags
OR Inclusive Or Logical; Motorola 680x0, Motorola 68300; performs a logical inclusive OR of a source operand with a destination operand and stores the result in the destination operand (8, 16, or 32 bits), one of the two operands must be a data register, the other operand can be the contents of any register or memory location; sets or clears flags
ORI Inclusive Or Immediate; Motorola 680x0, Motorola 68300; performs a logical inclusive OR of the contents of a register or memory location (8, 16, or 32 bits) with an immediate; sets or clears flags
OR Logical Inclusive OR; Intel 80x86; performs a logical OR between two registers, a register and contents of a memory location, or an immediate operand and either the contents of a register or the contents of a memory location; byte (8 bits), word (16 bits), or doubleword (32 bits); sets or clears 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
XOR Exclusive OR; DEC VAX; performs a logical exclusive 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) (XORB2 exclusive or byte 2 operand, XORB3 exclusive or byte 3 operand, XORW2 exclusive or word 2 operand, XORW3 exclusive or word 3 operand, XORL2 exclusive or longword 2 operand, XORL3 exclusive or longword 3 operand); sets or clears flags
EOR Exclusive Or Logical; Motorola 680x0, Motorola 68300; performs a logical exclusive or (XOR) of a source operand with a destination operand and stores the result in the destination operand (8, 16, or 32 bits), one of the two operands must be a data register, the other operand can be the contents of any register or memory location; sets or clears flags
EORI Exclusive Or Immediate; Motorola 680x0, Motorola 68300; performs a logical exclusive or (XOR) of the contents of a register or memory location (8, 16, or 32 bits) with an immediate; sets or clears flags
XOR Logical Exclusive OR; Intel 80x86; performs a logical exclusive OR between two registers, a register and contents of a memory location, or an immediate operand and either the contents of a register or the contents of a memory location; byte (8 bits), word (16 bits), or doubleword (32 bits); sets or clears 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
TST Test an Operand; Motorola 680x0, Motorola 68300; compares the contents of a register or memory location (8, 16, or 32 bits) with zero; sets or clears flags
TEST Logical Compare; Intel 80x86; compares the contents of a register or memory location (8, 16, or 32 bits) with an immediate value or the contents of a register; sets or clears flags
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
Scc Set According to Condition; Motorola 680x0, Motorola 68300; tests a condition code, if the condition is true then sets a byte (8 bits) of a data register or memory location to TRUE (all ones), if the condition is false then sets a byte (8 bits) of a data register or memory location to FALSE (all zeros): SCC, SCS, SEQ, SF, SGE, SGT, SHI, SLE, SLS, SLT, SMI, SNE, SPL, ST, SVC, SVS
SETcc Set Byte on Condition cc; Intel 80x86; tests a condition code, if the condition is true then sets a byte (8 bits) of a data register or memory location to TRUE (all ones), if the condition is false then sets a byte (8 bits) of a data register or memory location to FALSE (all zeros): SETA, SETAE, SETB, SETBE, SETC, SETE, SETG, SETGE, SETL, SETLE, SETNA, SETNAE, SETNB, SETNBE, SETNC, SETNE, SETNG, SETNGE, SETNL, SETNLE, SETNO, SETNP, SETNS, SETNZ, SETO, SETP, SETPE, SETPO, SETS, SETZ
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