PSW Program Status Word Addresses D0h Bit Addressable The Program Status Word

Psw program status word addresses d0h bit addressable

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interrupt the serial interrupt routine since the serial interrupt routine has the highest priority. PSW (Program Status Word, Addresses D0h, Bit-Addressable): The Program Status Word is used to store a number of important bits that are set and cleared by 8051 instructions. The PSW SFR contains the carry flag, the auxiliary carry flag, the overflow flag, and the parity flag. Additionally, the PSW register contains the register bank select flags which are used to select which of the "R" register banks are currently selected. ACC (Accumulator, Addresses E0h, Bit-Addressable): The Accumulator is one of the most used SFRs on the 8051 since it is involved in so many instructions. The Accumulator resides as an SFR at E0h, which means the instruction MOV A, #20h is really the same as MOV E0h,#20h . However, it is a good idea to use the first method since it only requires two bytes whereas the second option requires three bytes. B (B Register, Addresses F0h, Bit-Addressable): The "B" register is used in two instructions: the multiply and divide operations. The B register is also commonly used by programmers as an auxiliary register to temporarily store values. Basic Registers The Accumulator The Accumulator, as its name suggests, is used as a general register to accumulate the results of a large number of instructions. It can hold an 8-bit (1-byte) value and is the most versatile register the 8051 has due to the shear number of instructions that make use of the accumulator. More than half of the 8051’s 255 instructions manipulate or use the accumulator in some way. For example, if we add the number 10 and 20, the resulting 30 will be stored in the accumulator. 26
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The "R" registers The "R" registers are a set of eight registers that are named R0, R1, etc. up to and including R7. These registers are used as auxiliary registers in many operations. To continue with the above example, perhaps you are adding 10 and 20. The original number 10 may be stored in the Accumulator whereas the value 20 may be store in, say, register R4. To process the addition you would execute the command: ADD A,R4 After executing this instruction the Accumulator will contain the value 30. The "R" registers as very important auxiliary, or "helper", registers. The Accumulator alone would not be very useful if it were not for these "R" registers. The "R" registers are also used to temporarily store values. MOV A, R3; Move the value of R3 into the accumulator ADD A, R4; Add the value of R4 MOV R5, A; Store the resulting value temporarily in R5 MOV A, R; Move the value of R1 into the accumulator ADD A,R2 ;Add the value of R2 SUBB A,R5 ;Subtract the value of R5 (which now contains R3 + R4) In the above example we used R5 to temporarily hold the sum of R3 and R4. Of course, this isn’t the most efficient way to calculate (R1+R2) - (R3 +R4) but it does illustrate the use of the "R" registers as a way to store values temporarily.
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