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Negative flag
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In a computer processor the negative flag or sign flag is a single bit in a system status (flag) register used to indicate whether the result of the last mathematical operation produced a value in which the most significant bit (the left most bit) was set. In a two's complement interpretation of the result, the negative flag is set if the result was negative.
For example, in an 8-bit signed number system, -37 will be represented as 1101 1011 in binary (the most significant bit, or sign bit, is 1), while +37 will be represented as 0010 0101 (the most significant bit is 0).
The negative flag is set according to the result in the x86 series processors by the following instructions (referring to the Intel 80386 manual[1]):
- All arithmetic operations except multiplication and division;
- compare instructions (equivalent to subtract instructions without storing the result);
- Logical instructions – XOR, AND, OR;
- TEST instructions (equivalent to AND instructions without storing the result).
References
[edit]Negative flag
View on Grokipediafrom Grokipedia
The negative flag, also known as the sign flag (often abbreviated as N or SF), is a single-bit indicator in the status register (or flags register) of a computer's central processing unit (CPU) that signals whether the result of the most recent arithmetic, logical, or comparison operation is negative when interpreted as a signed integer in two's complement representation.[1][2] It is typically set to 1 if the most significant bit (MSB) of the result is 1, indicating a negative value, and cleared to 0 otherwise, thereby reflecting the sign of the outcome for signed number processing.[3]
This flag plays a crucial role in enabling efficient conditional branching and decision-making in assembly language programming, particularly for signed comparisons such as "branch if less than" (BLT) or "branch if greater than or equal" (BGE) instructions, where the negative flag, often in combination with the overflow flag, helps determine the relative order of signed operands without additional computations.[1][4] It is updated automatically by arithmetic instructions like addition and subtraction, as well as certain logical operations that affect the sign bit, though the specific instructions that update it vary by architecture.[2]
The negative flag has been a standard feature in numerous processor architectures since the early days of computing, including the Intel 8085 microprocessor (introduced in 1976), the MOS Technology 6502 (used in systems like the Commodore 64), ARM processors, and modern x86 architectures from Intel and AMD.[2][3] In these designs, it works in tandem with other status flags—such as the zero flag (Z), overflow flag (V or OF), and carry flag (C)—to provide comprehensive feedback on operation results, supporting both low-level optimization in embedded systems and high-level abstractions in compilers for signed integer handling.[1] Its implementation ensures portability across signed arithmetic routines while distinguishing signed from unsigned behaviors, a distinction critical for avoiding errors in numerical computations.[4]
) check the negative flag (N) against the overflow flag (V). The LT condition holds when N ≠ V, and GE when N = V, enabling signed branching in a single instruction following data-processing operations that update the condition flags.
In contrast, the 6502 processor employs simpler branching instructions BMI (branch if minus) and BPL (branch if plus), which directly test the negative flag (N) without incorporating overflow. BMI branches if N=1 (indicating a negative result), and BPL if N=0 (positive or zero). Programmers must manually account for overflow scenarios in signed comparisons, as the architecture lacks built-in integration of V with N for these branches.[10]
Overall, the use of the negative flag in these architectures facilitates efficient conditional branching for signed operations, often allowing single-instruction tests that avoid the need for explicit additional comparisons or multi-instruction sequences.[10]
Here, CMP subtracts BX from AX and sets SF if the result is negative, while JL (jump if less) branches if SF differs from the overflow flag (OF), confirming AX < BX in signed arithmetic. This approach ensures correct handling of negative ranges, such as distinguishing -1 from 255 in byte operations.[51]
In ARM assembly, the negative flag (N) is tested via conditional branch suffixes after a CMP instruction, which performs a subtraction and updates the application program status register (APSR). An example checks if a register value is less than a constant in signed terms:
The CMP sets N if R0 < 10 (signed), and BLT (branch if less than) executes if N differs from the overflow flag (V), directing flow to the loop label for values like -5 but not 15. This idiom is prevalent in embedded code for efficient decision-making in loops or error handling.[52]
For the 6502 processor, common in retrocomputing and emulators, the negative flag (N) is set by CMP based on bit 7 of the implied subtraction result, with BMI (branch if minus) providing a direct test for negative outcomes. A typical snippet loads a value and compares it to a threshold:
If the accumulator (50) is less than 100 in signed interpretation—though 50 is positive, this pattern shines for negative loads like LDA #$FF ( -1 ) versus #100, branching to negative_handler when N is set. This simplicity makes it ideal for resource-constrained 8-bit systems.[53]
Best practices emphasize combining the negative flag with the overflow flag for robust signed comparisons, as relying solely on N can fail in overflow scenarios; for example, in x86, use JL/JG over JS/JNS to account for both SF and OF, preventing errors like misclassifying overflowed positive results as negative. Conversely, avoid applying signed flag tests (e.g., BMI or JL) in unsigned contexts, where carry or zero flags should dominate to prevent interpreting high-bit values (like 255) as negative, which could lead to incorrect branching in counters or addresses.[54]
Despite the rise of high-level languages, the negative flag remains relevant in modern low-level programming, appearing in OS kernels for interrupt handlers and context switches, embedded systems for sensor data processing, and emulators optimizing retro software, where inline assembly ensures performance-critical signed operations without abstraction overhead.[54]
Overview
Definition
The negative flag, also known as the sign flag, is a single-bit indicator in a processor's flags register that reflects the most significant bit (MSB) of the result from the previous arithmetic or logical operation.[4] This bit position serves as a direct copy of the MSB from the operation's outcome, providing a simple mechanism to track the sign of the computed value.[5] In two's complement arithmetic, the negative flag signals whether the result is negative when the MSB is 1 or non-negative when the MSB is 0.[6] This interpretation aligns with the standard use of the MSB as the sign bit in signed integer representations.[7] The flag's general behavior is to be set to 1 if the MSB of the operation's result is 1 and cleared to 0 otherwise, while remaining unaffected by instructions that do not explicitly modify the flags register, such as data movement operations.[8] This ensures the flag retains its prior state unless updated by a relevant computation.[9]Purpose in Computing
The negative flag, often referred to as the sign flag, plays a crucial role in enabling processors to handle signed integers by distinguishing positive, zero, and negative results from arithmetic and logical operations without requiring additional instructions. It captures the most significant bit (sign bit) of the operation's outcome, typically in two's complement representation, allowing hardware to immediately signal whether the result represents a negative value. This mechanism ensures that signed computations can proceed efficiently at the hardware level, integrating seamlessly with the processor's arithmetic logic unit (ALU). For example, in ARM architectures, the N flag is set to the sign bit (bit 31 for 32-bit operations) if the result is negative, providing a direct indicator for signed integer processing.[1] Similarly, in the MOS Technology 6502, the negative flag reflects bit 7 of the result, facilitating straightforward sign detection in 8-bit signed arithmetic.[10] This flag further supports conditional execution and branching based on signed comparisons, such as determining if a value is less than (LT) or greater than or equal to (GE) another in signed contexts. By testing the N flag, processors can trigger jumps or alter program flow without computing the sign separately, optimizing control structures in code. In ARM processors, signed conditional branches like "bmi" (branch if minus, when N=1) or "bpl" (branch if plus, when N=0) rely on this flag to evaluate inequalities efficiently.[1] The 6502 employs analogous instructions, such as BMI (branch if minus) when the negative flag is set, enabling developers to implement signed decision logic with minimal overhead.[10] This capability is vital for algorithms involving comparisons, like sorting or conditional loops, where signed semantics must guide execution. The negative flag also facilitates efficient signed arithmetic in assembly programming and higher-level languages by permitting immediate post-operation inspection of results. After an ALU operation, software can query the flag to assess the sign, bypassing the need for bit manipulation or extra loads, which enhances performance in resource-constrained environments. In the 6502, for instance, instructions like ADC (add with carry) or CMP (compare) update the flag automatically, allowing assemblers to build robust signed routines with direct hardware feedback.[11] This integration reduces software overhead, making signed operations as streamlined as unsigned ones. Historically, the negative flag emerged as an essential feature in 1970s microprocessors to support signed number processing and emulate higher-level mathematical abstractions without excessive software intervention. Its design traces back to early systems like the Datapoint 2200 (1970), which included sign detection in its status logic, influencing subsequent chips such as Intel's 8008 (1972) and 8080 (1974). By the late 1970s, processors like the Intel 8086 (1978) and MOS Technology 6502 (1975) standardized this flag in their status registers, enabling efficient signed computations in the burgeoning era of personal and embedded computing.[12][11]Arithmetic Foundations
Two's Complement Representation
Two's complement is the predominant method for encoding signed integers in binary systems, utilizing the most significant bit (MSB) as the sign bit, where a value of 0 denotes a non-negative number (positive or zero) and 1 indicates a negative number. This representation allows for a symmetric treatment of arithmetic operations across positive and negative values within a fixed number of bits.[13][14] To represent a positive integer in two's complement, its binary equivalent is used directly, padded with leading zeros to the desired bit length if necessary. For negative integers, the process involves first expressing the absolute value in binary, then inverting all bits (complementing to one's complement), and finally adding 1 to the result. For instance, in an 8-bit system, the positive number 5 is represented as00000101. To obtain -5, invert to 11111010 and add 1, yielding 11111011. This method ensures that the negative value's binary pattern, when added to its positive counterpart, results in zero (modulo the bit width), facilitating efficient computations.[15][16]
In an n-bit two's complement system, the representable range spans from to , providing one more negative value than positive ones due to the absence of a distinct representation for . For an 8-bit example, this covers -128 to +127, as the pattern 10000000 uniquely denotes -128 while 00000000 is zero, and no separate encoding exists for +0 beyond this. This asymmetry arises inherently from assigning the MSB weight as negative, specifically .[17][18]
The adoption of two's complement stems from its simplification of hardware arithmetic: addition and subtraction operations mirror those for unsigned integers, requiring no special circuitry for sign handling, with results wrapping around modulo and status flags detecting anomalies like overflow. This uniformity reduces complexity in processor design and execution, making it the de facto standard in modern computing architectures.[18][19][20]
In the context of processor flags, the negative flag reflects the state of the sign bit in the two's complement result of an operation, signaling whether the outcome is negative.[15]
Sign Bit Operation
The negative flag directly reflects the most significant bit (MSB) of the result produced by an operation in two's complement representation, serving as an indicator of whether the result is negative, irrespective of the signs of the operands involved.[1][21] This mapping ensures that the flag captures the sign of the outcome post-calculation, providing a straightforward mechanism for signed value assessment in processor status registers.[1] In arithmetic operations such as addition and subtraction, the negative flag is set to 1 if the MSB of the result is 1, signifying a negative value, and cleared to 0 otherwise. For instance, performing 100 minus 150 in 8-bit two's complement arithmetic results in -50, represented as 11001110 binary, where the MSB of 1 sets the flag.[21][22] This behavior holds across operations that update the flag, treating the result uniformly under two's complement interpretation. Logical operations, including AND, OR, and XOR, also set the negative flag based on the MSB of their bitwise result, effectively treating the output as a signed integer.[21][1] For example, an AND operation yielding a result with MSB 1 will set the flag, mirroring the sign extension logic applied to arithmetic results. Certain edge cases arise in shift and rotate operations, where the negative flag's state depends on sign bit handling. Arithmetic shifts preserve the original sign bit by propagating it into vacated positions, ensuring the flag aligns with the resulting sign; for instance, a right arithmetic shift on a negative value maintains the MSB as 1, keeping the flag set.[23][24] Rotates, by contrast, may cycle bits without preservation, potentially altering the flag based on the new MSB configuration.[21]Implementations in Architectures
x86 Architecture
In the x86 architecture, the negative flag, known as the sign flag (SF), occupies bit 7 of the FLAGS register in 32-bit mode or the RFLAGS register in 64-bit mode. It reflects the most significant bit (MSB) of the result from an arithmetic, logical, comparison, or test operation, being set (1) if the MSB indicates a negative value in two's complement representation and cleared (0) otherwise. This behavior applies uniformly across operand sizes of 8, 16, 32, or 64 bits, with the 64-bit extension in x86-64 maintaining the same semantics but supporting larger RFLAGS operands via the REX.W prefix.[25] SF is updated by arithmetic instructions such as ADD, SUB, ADC, SBB, INC, DEC, and NEG, which compute the result and set SF based on its MSB, though multiplication (MUL, IMUL) and division (DIV, IDIV) instructions leave SF undefined. Logical instructions including AND, OR, XOR, and NOT similarly set SF to the MSB of the destination operand after the operation. The TEST instruction sets SF as if performing an AND without storing the result, while CMP performs a subtraction-like comparison to update SF without altering operands. Shift instructions like SAR (arithmetic right shift) preserve and propagate the sign bit, thereby setting SF to match the new MSB of the shifted value.[26][27][28][29][30][31] Explicit manipulation of SF occurs through dedicated instructions, as it lacks direct set or clear opcodes like those for the carry flag. The LAHF instruction loads the lower 8 bits of the FLAGS register (including SF) from the AH register, while SAHF stores bits from AH into the corresponding flags, allowing indirect control over SF. Broader access is provided by PUSHF/PUSHFD/PUSHFQ, which push the FLAGS or EFLAGS or RFLAGS onto the stack, and their POP counterparts, enabling full register save and restore operations that include SF. In x86-64, these instructions extend to 64-bit RFLAGS handling.[32][33][34]ARM Architecture
In ARM processors, the negative flag (N) resides in the Current Program Status Register (CPSR) for AArch32 execution state or the Application Program Status Register (APSR) for application-level operations, where it is set to 1 if the most significant bit (bit 31) of the instruction's result is 1, indicating a negative value when interpreted as a two's complement signed integer.[35][36] The N flag is updated by data processing instructions that affect the condition flags, including arithmetic operations such as ADD and SUB when suffixed with 'S' (e.g., ADDS, SUBS), logical operations like AND and ORR with the 'S' suffix (e.g., ANDS, ORRS), and comparison instructions like CMP, which always modify the flags based on the subtraction result without storing it.[37][38] In ARMv8 architecture with AArch64 execution state, the N flag is part of the NZCV condition flags register (accessible via system instructions like MRS), maintaining the same semantics as in earlier versions but applied to 64-bit operations, where it is set according to bit 63 of the result for signed integer interpretations.[39][40] This ensures compatibility for flag-based decisions in 64-bit contexts, with the APSR providing AArch32 compatibility views when needed. The N flag plays a key role in conditional execution through instruction suffixes in AArch32 (e.g., ADDS to both perform addition and set flags) and condition codes in both AArch32 and AArch64, enabling branches or operations like LT (signed less than), which executes if N ≠ V (where V is the overflow flag), thus supporting efficient signed comparisons without additional jumps.[41][42]6502 Processor
In the 6502 microprocessor, the negative flag (N) occupies bit 7 of the 8-bit Processor Status (P) register and serves as the sign indicator for the most significant bit (MSB) of arithmetic and logical results, where a value of 1 denotes a negative number in two's complement representation.[43][10] The flag is set to 1 if the MSB (bit 7) of the operation's result is 1 and cleared to 0 otherwise, mirroring the sign bit to facilitate signed comparisons and branching.[44] The N flag is affected by several instruction categories. Arithmetic operations such as ADC (add with carry) and SBC (subtract with carry) set N based on bit 7 of the accumulator result after the addition or subtraction, respectively.[10] Logical operations including AND (bitwise AND), EOR (exclusive OR), and ORA (bitwise OR) similarly update N to reflect bit 7 of the accumulator following the bitwise manipulation with the operand.[43] Compare instructions—CMP (compare accumulator), CPX (compare X register), and CPY (compare Y register)—set N according to bit 7 of the difference between the register and operand, effectively indicating the sign of the subtraction result.[10] Shift instructions also influence N: ASL (arithmetic shift left) sets it to the original bit 6 of the operand (which becomes the new bit 7 after shifting), while LSR (logical shift right) always clears N to 0, as the shift moves bit 7 out of the result.[43][10] A key nuance arises in decimal mode (enabled by the D flag in the P register), where ADC and SBC perform binary-coded decimal (BCD) arithmetic but set the N flag based on bit 7 of the raw binary intermediate result prior to any BCD correction, rather than the final adjusted value in the accumulator.[45] For instance, an ADC operation yielding a binary result with bit 7 set to 1 will set N, even if subsequent BCD adjustment flips that bit for the stored decimal value. This behavior holds for the original NMOS 6502 but differs in CMOS variants like the 65C02, where N reflects bit 7 after full BCD correction.[46][45] The 65C02, an enhanced version of the 6502, introduces additional instructions such as bit manipulation extensions and new addressing modes but maintains consistent N flag behavior for core arithmetic, logical, compare, and shift operations in binary mode, with the noted decimal mode variation.[10][45]Interactions and Comparisons
Relation to Overflow Flag
The negative flag (N) and the overflow flag (V or OF) play complementary yet distinct roles in handling signed arithmetic within processor status registers. The negative flag directly mirrors the most significant bit (sign bit) of the arithmetic result, indicating whether the outcome represents a negative value (N=1) or a non-negative value (N=0) in two's complement representation.[47] In contrast, the overflow flag signals an error in signed arithmetic when the result exceeds the representable range, such as producing an incorrect sign due to magnitude overflow, but it does not reflect the sign itself.[48] For addition in two's complement, the overflow flag is set if the carry into the sign bit differs from the carry out of the sign bit, computed as their XOR; this is equivalent to the condition where the operands have the same sign but the result has a different sign.[48] This detection can also be expressed using sign bits as V = (sign_A XOR sign_B) AND (sign_A XOR sign_result), highlighting how overflow arises specifically from sign inconsistencies rather than merely the result's sign.[48] For example, in an 8-bit system, adding +127 (01111111) and +1 (00000001) yields 10000000 (-128), setting V=1 because both inputs are positive but the result appears negative, invalidating the signed operation.[48] The negative flag, however, remains agnostic to such validity issues and simply reports the result's sign. A valid negative result, such as subtracting 100 from 50 to get -50 (11100010 in 8-bit two's complement), sets N=1 while leaving V=0, as no overflow occurs.[47] This distinction ensures that N provides raw sign information for immediate use, whereas V requires checking to confirm arithmetic integrity. In signed comparisons, the flags are combined to achieve accurate relational tests that account for potential overflow. The condition for less-than (LT) is true when N XOR V = 1, which corrects for overflow scenarios by effectively using the "effective sign bit" (N XOR V) to determine the true ordering of signed operands.[49] This approach, common in architectures like x86 and PDP-11, prevents misinterpretation of results tainted by overflow.[49]Relation to Zero Flag
The negative flag (N) and zero flag (Z) complement each other in analyzing operation results within CPU status registers, particularly in two's complement arithmetic and logical computations. The zero flag is set to 1 if the entire result is zero—all bits are 0—indicating equality or completion of a sequence, such as in subtraction or logical AND operations. In contrast, the negative flag is set to 1 if the most significant bit (MSB) of the result is 1, signifying a negative value in signed integer representations.[50] This distinction allows N to focus solely on the sign bit, ignoring lower bits, while Z requires verification across all bits. In standard two's complement systems, the representation of zero is always the all-zero bit pattern, with the MSB cleared (0), ensuring that N=0 whenever Z=1. Both flags cannot be set simultaneously, as a negative zero does not exist distinctly; any apparent -0 is treated as positive zero. This behavior holds across architectures like ARM and x86, where instructions such as ADD, SUB, or CMP update both flags based on the result without overlap in the zero case.[50] For combined usage, the zero flag serves as the primary indicator for equality checks (e.g., comparing two values yields Z=1 if identical), while the negative flag provides sign information only when Z=0, revealing whether a non-zero result is positive or negative. In logical operations like AND, OR, or XOR, both flags are updated analogously: for instance, ANDing any value with 0 produces an all-zero result, setting Z=1 and N=0, as the MSB cannot be 1 in a zero pattern.[50] This interplay supports efficient result analysis in algorithms, such as loop terminations where Z detects zero crossings and N assesses directional signs in signed magnitudes, without redundant bit inspections.Applications
Conditional Branching
The negative flag, often denoted as N or SF in various architectures, plays a crucial role in enabling conditional branching based on signed arithmetic conditions in assembly language programming. Following arithmetic operations such as subtraction (SUB or CMP), the negative flag is set if the most significant bit of the result indicates a negative value in two's complement representation. This allows processors to perform branches that reflect signed comparisons, such as determining if one value is less than or greater than or equal to another, by examining the negative flag in conjunction with other status flags.[10] In general, signed branching instructions like BLT (branch if less than) evaluate to true when the negative flag differs from the overflow flag (N ≠ V), while BGE (branch if greater than or equal) branches when they are the same (N = V). These conditions arise after a comparison operation that sets the flags appropriately, providing a mechanism to implement signed inequalities without additional computations. This approach ensures that the sign extension behavior in two's complement arithmetic is correctly interpreted for control flow decisions. In the x86 architecture, conditional jump instructions such as JL (jump if less than) and JGE (jump if greater than or equal) utilize the sign flag (SF, equivalent to N) XORed with the overflow flag (OF, equivalent to V) to determine the branch. Specifically, JL branches if SF ≠ OF, indicating a negative result from a signed subtraction, while JGE branches if SF = OF. This integration allows for efficient signed comparisons directly from the flags set by prior CMP instructions. Similarly, in ARM architectures, conditional execution suffixes like LT (less than) and GE (greater than or equal) for branch instructions (e.g., BUsage in Assembly Programming
In assembly programming, the negative flag, often denoted as SF in x86 or N in ARM and 6502 architectures, serves as a key indicator for the sign of arithmetic results, enabling efficient conditional control flow for signed operations. Programmers leverage it primarily after comparison or arithmetic instructions to branch based on whether a value is negative, facilitating tasks like signed magnitude checks or inequality tests without additional computations. This flag's behavior aligns with two's complement representation, where it reflects the most significant bit of the result.[51] A common pattern in x86 assembly involves using the CMP instruction to set flags, followed by a conditional jump that incorporates the sign flag for signed comparisons. For instance, to branch if one register holds a smaller signed value than another:CMP AX, BX
JL label
CMP AX, BX
JL label
CMP R0, #10
[BLT](/page/BLT) loop
CMP R0, #10
[BLT](/page/BLT) loop
LDA #50
CMP #100
BMI negative_handler
LDA #50
CMP #100
BMI negative_handler