Each status flag bit in the status register is set based on the "last operation result of the ALU"; the status register, also known as the condition code register, is part of the computer's core component arithmetic unit and is mainly used to Stores various status information and control information that reflect the current instruction execution results.
The operating environment of this tutorial: Windows 10 system, DELL G3 computer.
The status flag bits in the status register are based on the "last operation result of the ALU" The
status register that sets the flag bit is also called the condition code register. It is the core component of the computer system - part of the arithmetic unit. The status register is used to store two types of information:
One type is various status information (condition codes) that reflect the current instruction execution results, such as whether there is a carry (CF bit), whether there is an overflow (OV bit), whether the result is positive or negative (SF bit), whether the result is zero (ZF bit) ), parity flag bit (P bit), etc.;
The other type is to store control information (PSW: program status word register), such as interrupt enable (IF bit), tracking flag (TF bit), etc. In some machines, the PSW is called the flag register FR (Flag Register)
There are 6 status registers in ARM, and the ARM7TDMI core contains 1 CPSR and 5 SPSR for exception handlers. CPSR reflects the current status of the processor, which contains:
4 condition code flags (negative (N), zero (Z), carry (C) and overflow (V)) ;
2 interrupt disable bits, each controlling a type of interrupt;
5 bits encoding the current processor mode ;
1 bit used to indicate the currently executing instruction (ARM or Thumb).
Explanation of each flag bit of PSW:
1. CY (Carry): used to represent the carry in addition operations and the carry in subtraction operations Borrow, if there is a carry in the addition operation or a borrow in the subtraction operation, the CY position is 1, otherwise it is 0
2. AC (Auxiliary Carry): It is basically the same as CY, except that the lower 4 bits are the same as CY. Operations between the high 4 bits.
3.F0 (Flag Zero user flag bit): This bit is a flag bit set by the user according to his own needs. The user can determine the flow and branch of the program by setting this bit.
4. RS1, RS0: 8051 has eight 8-bit working registers R0~R7, and its actual physical location in RAM can be selected and determined according to needs.
00：00H~07H
01：08H~0FH
10：10H~17H
11：18H~1FH
5.OV: Indicates whether an overflow occurs during the operation. If the result exceeds the range of data that can be represented by an 8-bit binary number, that is, a signed number -128~127, the flag position is 1.
6.OP: The parity flag bit is used to indicate the parity of the number of 1's in the operation result. If P=0, then the number of 1's in the accumulator A is an even number; if P=1, then The number of 1's in accumulator A is an odd number.
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