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assembly language step by step programming with dos and linux PHẦN 5

Chương trước đã thảo luận về các công cụ được sử dụng để xuất bản các dịch vụ web từ quan điểm của một tác giả dịch vụ Web. Chương này xem xét các công cụ được sử dụng để tìm các dịch vụ web từ quan điểm của một người tiêu dùng dịch vụ Web. | On the next page is a sample entry from Appendix A. Refer to it during the following discussion The instruction s mnemonic is at the top of the page highlighted in a box to make it easy to spot while flipping quickly through the appendix. To the mnemonic s right is the name of the instruction which is a little more descrip-tive than the naked mnemonic. Micro Chart Micro Logic Corp. P.O. Box 174 Hackensack NJ 07602 Neg Negate two s complement multiply by -1 Flags affected O D I T S Z A P C OF Overflow flag F F F F F F F F F DF Direction flag IF Interrupt flag TF Trap flag SF Sign flag ZF Zero flag AF Aux carry PF Parity flag CF Carry flag Legal forms NEG r8 NEG m8 NEG r16 NEG m16 Examples NEG AL NEG CX NEG BYTE PTR BX NEG WORD PTR DI Negates byte quantity at DS BX Negates word quantity at DS BX Notes This is the assembly-language equivalent of multiplying a value by -1. Keep in mind that negation is not the same as simply inverting each bit in the operand. Another instruction NOT does that. The process is also known as generating the two s complement of a value. The two s complement of a value added to that value yields zero. -1 FF -2 FE -3 FD etc. If the operand is 0 CF is cleared and ZF is set otherwise CF is set and ZF is cleared. If the operand contains the maximum negative value -128 for 8-bit or -32768 for 16-bit the operand does not change but OF and CF are set. SF is set if the result is negative or cleared if not. PF is set if the low-order 8 bits of the result contain an even number of set 1 bits otherwise PF is cleared. NOTE You must use a type override specifier BYTE PTR or WORD PTR with memory data r8 AL AH BL BH CL CH DL DH r 6 AX BX CX DX BP SP SI DI sr - CS DS SS ES m8 8-bit memory data m16 - 16-bit memory data i8 - 8-bit immediate data i 6 16-bit immediate data d8 8 bit signed displacement d16 16-bit signed displacement Flags Immediately beneath the mnemonic is a minichart of machine flags in the Flags register. I haven t spoken in detail of flags .