各种标志位的含义（The meaning of various flag bits）

各种标志位的含义（The meaning of various flag bits） 各种标志位的含义(The meaning of various flag bits) The meaning of various flag bits Arithmetic result flag bit 1, carry flag CF (Carry Flag) The carry flag CF is mainly used to reflect the operation whether or carry out. If the high operation result produced a carry or borrow, then its value is 1, otherwise the value is 0. The use of the flag bits include: the addition and subtraction of multiple word (byte) numbers, the size of unsigned numbers, comparison operations, shift operations, shifts between words (bytes), instructions to change CF values, and so on. 2, parity flag PF (Parity Flag) The parity flag PF is used to reflect the parity of the number of "1" in the operation result. If the number of "1" is even, the value of "PF" is 1, otherwise the value is 0. Parity check can be performed with PF or parity bit is generated. In the data transfer process, in order to provide the reliability of the transmission, if the parity check method is used, the flag bit can be used. 3, auxiliary carry flag AF (Auxiliary, Carry, Flag) When the following occurs, the value of the auxiliary carry flag AF is set to 1, otherwise the value is 0: In a word, (1) operation, low byte to byte carry or borrow at high; (2), in bytes operation, low 4 bits high 4 bits carry or borrow when. For the above 6 arithmetic flag bits, the flag bits CF, ZF, SF, and OF are used more frequently in general programming, while the bit flags PF and AF are less frequently used. 4, zero mark ZF (Zero Flag) The zero flag ZF is used to reflect whether the result is 0. If the result is 0, the value is 1, otherwise the value is 0. This flag is used when the result of the calculation is 0. 5, symbol SF (Sign Flag) A symbol bit used to reflect the result of an operation, SF, which is the same as the highest point of the result of an operation. In a microcomputer system, signed numbers are represented by a complement notation, so SF also reflects the positive and negative numbers of the results. When the result is positive, the value of SF is 0, otherwise the value is 1. 6, overflow flag OF (Overflow Flag) The overflow flag OF is used to reflect whether or not the result of the unsigned addition and subtraction operation overflows. If the result exceeds the range represented by the current number of bits, it is called overflow, and the value of OF is set to 1, otherwise the value of OF is cleared to 0. "Overflow" and "carry" are two different meanings. Don't confuse them. If you are not quite sure, please refer to the relevant sections in the course of computer composition principles. Two status control flag bit The status flag is used to control the CPU operation, which is changed by special instructions. 1, tracking mark TF (Trap Flag) When the trace flag TF is set to 1, the CPU enters the single step mode, i.e., each execution of the instruction generates a single step interrupt request. This method is mainly used for debugging programs. There is no special instruction in the instruction system to change the value of the flag bit TF, but programmers can change its value by other means. 2 interrupt enable flag IF (Interrupt-enable Flag) The interrupt enable flag IF is used to determine whether the CPU responds to an interrupt request issued by an CPU that is blocked by an external interrupt. Regardless of the value of the flag, the CPU must respond to an interrupt request from an CPU that is not blocked by an external interrupt, as well as an interrupt request generated within the CPU. Specific provisions are as follows: (1) when IF=1, the CPU can respond to interrupts requested by a masked interrupt outside of CPU; (2) when IF=0, CPU does not respond to interrupts requested by a masked interrupt outside of CPU. CPU's instruction system also has special instructions to change the value of the flag bit IF. 3, direction sign DF (Direction Flag) The direction flag DF is used to determine the direction in which pointer registers are adjusted during execution of a string operation instruction. Specific provisions are given in section 5.2.11 - string manipulation instructions. In the microcomputer instruction system, special instructions are also provided to change the value of the flag bit DF. Three, 32 bit flag register added bit 1、I/O特权标志而且(I/O特权级) I/O特权标志用两位二进制位来表示，也称为I/O特权级字段。该字 段指定了要求执行I/O指令的特权级。如果当前的特权级别在数值上 小于等于而且的值，那么，该I/O指令可执行，否则将发生一个保护 异常。 2、嵌套任务标志NT(嵌套任务) 嵌套任务标志NT用来控制中断返回指令iret具体 规定 关于下班后关闭电源的规定党章中关于入党时间的规定公务员考核规定下载规定办法文件下载宁波关于闷顶的规定 如下的执行: (1)、当NT = 0，用堆栈中保存的值恢复eflags、CS和EIP，执行常规的中断返回操作; (2)、当NT = 1，通过任务转换实现中断返回。 3、重启动标志射频(启动标志) 重启动标志射频用来控制是否接受调试故障。规定:RF = 0时，表示”接受”调试故障，否则拒绝之。在成功执行完一条指令后，处理机把射频置为0，当接受到一个非调试故障时，处理机就把它置为1。 4、虚拟8086方式标志VM(虚拟8086模式) 如果该标志的值为1，则表示处理机处于虚拟的8086方式下的工作状态，否则，处理机处于一般保护方式下的工作状态。