Category: Status flags in 8085

In microprocessor, the flags register can have a total of eight flags. Thus a flag can be represented by 1 bit of information. But only five flags are implemented in And they are:. The respective position of these flag bits in flag register has been show the below figure.

The user is not required to memorize the positions of these flags in the flags register.

status flags in 8085

Now consider the programmer's view of contains the flags register has been depicted in the following figure. These individual flags are either set to 1, or reset to 0 depending on the result of execution of the last executed arithmetic or logical instruction. But in a few arithmetic and logical instructions, some or none of these flags are affected. Also there are some arithmetic and logical instructions, flag bits in the flag register will not get affected as well.

However, in any data transfer instruction, none of the flags bits in flag register are affected. Now let us consider each flag bit separately for our further discussions. Carry flag Cy : after performing the addition of any two 8-bit numbers, the carry generated can be either 0 or 1. That is only 1-bit. Thus to store the carry information 1-bit storage is enough. The Cy flag is stored in the LS bit position in the flags register.

Instructions that use the Cy flag are widely used in the user programs.

Flag register in 8085 microprocessor

Auxiliary carry flag Ac : Now let us consider the addition of any two 8-bit 2-hex digit numbers, a carry may be generated when we add the LS hex digits of the two numbers. Such a carry is called intermediate carry also known as half carry, or auxiliary carry AC.

Intel prefers to call it AC. As this is only an intermediate carry, we may not be interested in storing this bit information. But microprocessor still stores this AC information in bit position 4 of the flags register. The result of execution of DAA instruction, is affected by the status of this flag. However, in our instruction set does not provide any instruction, which explicitly uses the AC flag.

Sign flag S : The S flag is set to 1, when the result thus produced against any logical or arithmetic operations is negative, indicated by MS bit of 8-bit result being 1. It is reset to 0 otherwise if the result is positive, indicated by MS bit of 8-bit result being 0. Thus, the value of S flag is essentially the value of the MS bit of the 8-bit result.

Note that we are not considering here the 9-bit result including the carry, to decide the S flag value. But when we shall work with unsigned numbers, then we shall simply ignore the S flag. For example, if we are treating 85H and 1EH as unsigned numbers, their sum will be the unsigned number A3H. In this case, S flag becomes 1, but we do not care for the value of the S flag.

status flags in 8085

And we shall ignore it as well. Parity flag P : The P flag is set to 1, if the 8-bit result thus produced against any logical and arithmetic operation has an even number of 1's in it.

If there are odd number of 1's in the 8-bit result, the P flag is reset to 0. As the user does not really care for the number of 1's present in the result after an arithmetic operation, this flag is not of much use practically. Zero flag Z : The Z flag is set to 1, if after arithmetic and logical operations, the 8-bit result thus produced, is 00H. If the 8-bit result is not equal to 00H, the Z flag is reset to 0.

Thus the Z flag is hoisted to indicate that the result is 0. In previous Example 1, as the 8-bit result is 38H and is non-zero, the Z flag is reset to 0.A status registerflag registeror condition code register CCR is a collection of status flag bits for a processor.

The status register is a hardware register that contains information about the state of the processor. The status register lets an instruction take action contingent on the outcome of a previous instruction.

Typically, flags in the status register are modified as effects of arithmetic and bit manipulation operations. For example, a Z bit may be set if the result of the operation is zero and cleared if it is nonzero. Other classes of instructions may also modify the flags to indicate status. The flags are read by a subsequent conditional instruction so that the specified action depending on the processor, a jump, call, return, or so on occurs only if the flags indicate a specified result of the earlier instruction.

Such machines either do not pass implicit status information between instructions at all, or they pass it in an explicitly selected general purpose register. A status register may often have other fields as well, such as more specialized flags, interrupt enable bits, and similar types of information. During an interrupt, the status of the thread currently executing can be preserved and later recalled by storing the current value of the status register along with the program counter and other active registers into the machine stack or some other reserved area of memory.

This is a list of the most common CPU status register flags, implemented in almost all modern processors. Status flags enable an instruction to act based on the result of a previous instruction.

In pipelined processors, such as superscalar and speculative processors, this can create hazards that slow processing or require extra hardware to work around them. Some very long instruction word processors dispense with the status flags. A single instruction both performs a test and indicates on which outcome of that test to take an action, such as Compare a with b and Jump to c if Equal. The result of the test is not saved for subsequent instructions. Another alternative to the status register is for processor instructions to deposit status information in a general-purpose register when the program requests it.

Conditional branches act based on the value in the general-purpose register. To test for other conditions, a program uses an equivalence formula. For example, MIPS has no "carry bit" but a program performing multiple-word addition can test whether a single-word addition of registers overflowed by testing whether the sum is lower than an operand: [4]. The sltu instruction sets tmp to 1 or 0 based on the specified comparison of its two other operands.

Here, the general-purpose register tmp is not used as a status register to govern a conditional jump; rather, the possible value of 1, indicating carry from the low-order addition, is added to the high-order word.

Fortunately, those two carries may be added to each other without risk of overflow, so the situation stabilizes at five instructions per word added. From Wikipedia, the free encyclopedia. Retrieved Microchip Technology. Categories : Control flow Central processing unit Digital registers. Namespaces Article Talk. Views Read Edit View history. By using this site, you agree to the Terms of Use and Privacy Policy. Indicates that the result of an arithmetic or logical operation or, sometimes, a load was zero.

It is also used to extend bit shifts and rotates in a similar manner on many processors sometimes done via a dedicated X flag.

Indicates that the result of a mathematical operation is negative. In some processors, [2] the N and S flags are distinct with different meanings and usage: One indicates whether the last result was negative whereas the other indicates whether a subtraction or addition has taken place.

Indicates that the signed result of an operation is too large to fit in the register width using two's complement representation. Half-carry flag Auxiliary flag Digit Carry Decimal adjust flag.If we can't tunnel through the Earth, how do we know what's at its center? All Rights Reserved. The material on this site can not be reproduced, distributed, transmitted, cached or otherwise used, except with prior written permission of Multiply.

Hottest Questions. Previously Viewed. Unanswered Questions. Explain different types of flags in microprocessor? Wiki User Zero flag : if result is 0 than it is set condition i.

Flag register of 8086 microprocessor

Carry flag : if an extra bit arises than it is set condition I. Sign flag : in it only D7 is considered. Parity flag : if number of 1's in the answer is even than it Is set condition i. Related Questions Asked in IntelIntel and Explain the different types of addressing modes in microprocessor? Asked in Microprocessors, Intel Types of instruction in microprocessor? Asked in Microprocessors, IntelIntel and What are the different types of microprocessor?

Flags register in 8085 Microprocessor

Asked in Microprocessors What are the different types of microprocessor chips? Asked in Business and Industry Explain characteristics of different types of organsational structures in management function and behavior? Explain different types of ability. Asked in Science What are the types of microprocessor register? Microprocessor consists of different types of registers. They special purpose registers, general purpose registers, address registers, floating point registers and constant registers.

Asked in Microprocessors Name three different types of microprocessor system application? IndustrialCommerical and Domestic.

Asked in Intel What are the various types of interrupts in microprocessor? The fluctuation in the power surges is an example of the types of interrupts in a microprocessor. There are six different types of computers in the world such as the microprocessor. Another type of computer is the personal computers. There are currently four different types of flags being used in the UK at this very moment in time.

Asked in Computers, Technology What are the different types of computing and explain them?

Status or Flag Register in 8085 Microprocessor

Asked in Intel Types of sorting in microprocessor? Sorting is not a microprocessor specific thing. Sorting requires a program and, as such, is not dependent on which microprocessor is involved.

Asked in Flags What types of flags can be flown at veterans memorials? American flags, pow mia flags. Asked in C Programming Explain the different types of constants n variables in C? Explain data model? Asked in Business and Industry Explain characteristics of different types of organsational structures in management function and behaviour? Answers for explain the different characteristics a business.The FLAGS register is the status register in Intel x86 microprocessors that contains the current state of the processor.

The wider registers retain compatibility with their smaller predecessors. The fixed bits at bit positions 1, 3 and 5, and carry, parity, adjust, zero and sign flags are inherited from an even earlier architecture, and The adjust flag used to be called auxiliary carry bit in and half-carry bit in the Zilog Z80 architecture. All FLAGS registers contain the condition codesflag bits that let the results of one machine-language instruction affect another instruction.

Arithmetic and logical instructions set some or all of the flags, and conditional jump instructions take variable action based on the value of certain flags. Other conditional jumps test combinations of several flags. FLAGS registers can be moved from or to the stack.

This is part of the job of saving and restoring processor context, against a routine such as an interrupt service routine whose changes to registers should not be seen by the calling code.

Here are the relevant instructions:. For example, the cld and std instructions clear and set the direction flag DFrespectively; but there is no instruction to complement DF. This can be achieved with the following assembly code :.

For example, the alignment flag can only be changed on the and above. If the program tries to modify this flag and senses that the modification did not persist, the processor is earlier than the However, the above method remains useful to distinguish between earlier models. From Wikipedia, the free encyclopedia. May Dec Retrieved Categories : Digital registers X86 architecture. Hidden categories: Wikipedia articles needing clarification from November Namespaces Article Talk.Intel is an 8-bit, NMOS microprocessor.

It is a 40 pin C package fabricated on a single LSI chip. C supply for its operation. Its clock speed is about 3 MHz. The clock cycle is of ns.

The time for the back cycle of the Intel A-2 is ns. It has 80 basic instructions and opcodes. Figure 1 shows the block diagram of Intel A. It consists of three main sections, an arithmetic and logic unit a timing and control unit and several registers. These important sections are described as under.

The timing and control unit is a section of the CPU. It generates timing and control signals which are necessary for the execution of instructions. It controls the entire operation of the microprocessor and peripherals consented to it. Thus it is seen that control unit of the CPU acts as a brain of the computer.

Figure 1 show the various registers of Intel A. Registers are small memories within the CPU.

status flags in 8085

They are used by the microprocessor for temporary storage and manipulation of data and instructions. In a large computer the number of registers is more and hence the program requires less transfer of data to and from the memory. In small computers the number of registers is small due to the limited size of the chip. Intel microprocessor has the following registers.

Figure 1 the block diagram of Intel The program counter PC, contains the address of the next instruction.If we can't tunnel through the Earth, how do we know what's at its center? All Rights Reserved. The material on this site can not be reproduced, distributed, transmitted, cached or otherwise used, except with prior written permission of Multiply. Hottest Questions. Previously Viewed. Unanswered Questions. Intel We need you to answer this question! If you know the answer to this question, please register to join our limited beta program and start the conversation right now!

Related Questions Asked in Microprocessors Status signal in microprocessor ? Asked in Microprocessors, Intel What is flags in microprocessor? Flags are microprocessor dependent. These flags can also be used for logic implementation. Asked in Intel What are flags in microprocessor ? Asked in History, Flags What are the various flags used in ? All flags are affected after the SUB operation to reflect the result of subtraction.

Asked in Flags, Intel What are the meaning of flags in microprocessor? Asked in Intel Which are the flags in microprocessor? Asked in Flags, IntelIntel and What is flag register of ? The flag register is in an microprocessor. It is an 8-bit register which contains 5 bit positions.

These flags are known as zero, sign, carry, parity and auxiliary carry. Asked in IntelIntel andThe Difference Between What is the difference between compare and subtract instructions in ? The compare and subtract instructions in the both subtract one operand from another, and set flags accordingly.Like most microprocessors, the has a flag register that holds status information on the results of an operation. The flag register is 8 bits: bit 0 holds the carry flag, bit 2 holds the parity, bit 3 is always 0, bit 4 holds the half-carry, bit 6 holds the zero status, and bit 7 holds the sign.

But what about the missing bits: 1 and 5? Back inusers of the determined that these flag bits had real functions. But it has a totally diffrent value for arithmetic operations.

status flags in 8085

As I will show, that published description is mistaken. The K flag actually is the V flag exclusive-ored with the sign of the result. And the purpose of the K flag is to compare signed numbers. The circuit for the K and V flags The following schematic shows the reverse-engineered circuit for the K and V flags in the The V flag is simply the exclusive-or of the carry into the top bit and the carry out of the top bit. This is a standard formula for computing overflow [2] for signed addition and subtraction.

The computes the same overflow value through different logic. The V flag has values for other arithmetic operations, but the values aren't useful.

Finally, a tri-state superbuffer the large triangle writes the flag value to the bus when needed. The K flag circuitry is on the right. The final function of the K flag is the result of a signed comparison. The K flag is the exclusive-or of the V flag and the sign bit of the result. For subtraction and comparison, the K flag is 1 if the second value is larger than the first. The flags are generated from the carries and results from the ALU. One mystery was the purpose of the K flag: "It does not resemble any normal flag bit.

It turns out the the K flag is useful for signed comparisons. If you're comparing two signed values, the first is smaller if the exclusive-or of the sign and overflow is 1. From the circuit above, it is clear that the V and K flags were deliberately added to the chip.

This is in contrast to thewhere undocumented opcodes have arbitrary results due to how the circuitry just happens to work for unexpected inputs. My theory is that Intel decided they didn't want to support K or 8-bit V flags in theso in order to make the source-compatible with thethey dropped those flags from the documentation, but the circuitry remained in the chip.

The remainder of this article will show how the V and K flag circuits work, diving all the way down to the silicon circuits. The above image of the chip shows the layout of the chip and the components that are important to the discussion. In the upper left of the chip is the ALU arithmetic-logic unitwhere computations happen details.

The data bus is the main interconnect in the chip, connects the data pins upper leftthe ALU, the data registers, the flag register, and the instruction decoding upper right. In the lower left of the chip is the bit register file. For some reason, the ALU has the low-order bit on the right, while the registers have the low-order bit on the left. The flag logic circuitry sits underneath the ALU, with high-current drivers right on top of the data bus.

The flags are arranged in apparently-random order with bit 7 sign on the left and bit 6 zero on the right.