How To Reset Flag Register In 8085

Flags register in 8085 Microprocessor

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In 8085 microprocessor, the flags register can have a total of eight flags. Thus a flag tin be represented past 1 bit of information. But just five flags are implemented in 8085. And they are:

• Carry flag (Cy),

• Auxiliary carry flag (Air conditioning),

• Sign flag (S),

• Parity flag (P), and

• Zero flag (Z).

The respective position of these flag bits in flag register has been show the below figure. The positions marked by "ten" are to exist considered every bit don't care bits in the flags register. The user is not required to memorize the positions of these flags in the flags register.

Fig. Flags register

At present consider the programmer's view of 8085 contains the flags register has been depicted in the following figure -

Fig. Programmer's view of 8085 Flags register

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 education. 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 volition not go affected as well. As case in the execution of DCX and INX instructions, flag bit in flag register will non become affected at all.

Nonetheless, in any data transfer instruction, none of the flags bits in flag register are affected. At present let u.s. consider each flag bit separately for our further discussions.

Carry flag (Cy): later on performing the add-on of any ii eight-scrap numbers, the acquit generated tin exist either 0 or 1. That is only one-flake. Thus to shop the comport 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.

Example 1: In the addition of 45H and F3H, the upshot thus produced volition exist 38H and with Cy flag = i, as shown below.

Example two: In the addition of 85H and 1EH, the effect thus produced will exist A3H with Cy = 0, as shown below.

Auxiliary carry flag (Air-conditioning): Now let us consider the addition of any 2 8-bit (2-hex digit) numbers, a carry may be generated when nosotros add together the LS hex digits of the two numbers. Such a comport is called intermediate deport also known as one-half deport, or auxiliary comport (Air conditioning). Intel prefers to call it Air conditioning. In the in a higher place Example 1, AC was not generated but in Instance 2, Air-conditioning is generated.

Equally this is only an intermediate acquit, nosotros may not be interested in storing this bit information. But 8085 microprocessor still stores this Air conditioning information in chip position 4 of the flags annals. The result of execution of DAA instruction, is afflicted by the status of this flag. However, in our 8085 instruction set does not provide whatever instruction, which explicitly uses the Air conditioning flag.

Sign flag (S): The South flag is set up to one, when the result thus produced against any logical or arithmetic operations is negative, indicated by MS bit of 8-bit result existence 1. It is reset to 0 otherwise if the result is positive, indicated by MS fleck of 8-flake result being 0.

Thus, the value of S flag is essentially the value of the MS scrap of the 8-bit effect. In the to a higher place Case i, every bit the 8-bit effect is 38H = 0 011 1000, 0 in MSB indicates result is positive and the sign flag is reset to 0. Notation that we are non considering here the 9-bit result including the carry, to decide the S flag value. In Example two, as the 8-flake result is A3H = 1 010 0011, the MSB has become 1 that means negative and the sign flag is ready to i.

Simply when we shall work with unsigned numbers, so we shall only ignore the S flag. For case, if we are treating 85H and 1EH as unsigned numbers, their sum will exist the unsigned number A3H. In this instance, S flag becomes 1, but we do not care for the value of the S flag. And nosotros shall ignore it as well.

Instructions that apply the Southward flag are quite ofttimes used in the user programs.

Parity flag (P): The P flag is prepare to 1, if the 8-bit result thus produced against any logical and arithmetic performance has an even number of i's in it. If in that location are odd number of 1's in the eight-bit result, the P flag is reset to 0.

In our previous Case ane, as the 8-scrap result 38H = 0011 1000 has iii numbers of 1's, then having odd number of 1's, the parity flag is reset to 0. On the other hand, in Example ii, as the 8-scrap outcome A3H = 1010 0011 has iv numbers of ane's (then an even number of one's), the parity flag is prepare to 1.

Every bit the user does non really care for the number of 1's nowadays in the event after an arithmetic performance, this flag is not of much use practically.

Zero flag (Z): The Z flag is ready to i, if after arithmetic and logical operations, the 8-bit issue thus produced, is 00H. If the 8-bit result is non equal to 00H, the Z flag is reset to 0. Thus the Z flag is hoisted to betoken that the result is 0.

In previous Example i, as the viii-bit result is 38H and is non-zippo, the Z flag is reset to 0. Besides in the other hand in Example 2, as the 8-flake effect is A3H, the Z flag is reset to 0 here once again. Instructions that utilise the Z flag are widely used in the user programs.

Published on 28-Feb-2019 11:55:38

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How To Reset Flag Register In 8085,

Source: https://www.tutorialspoint.com/flags-register-in-8085-microprocessor

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