Assembly instructions of STOS, REP

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STOS instructions: speaking Al / AX / EAX value stored in the [EDI] memory cell designated

　　STOS BYTE PTR ES: [EDI] abbreviation STOSB

　　STOS WORD PTR ES: [EDI] abbreviation STOSW

　　STOS DWORD PTR ES: [EDI] abbreviation STOSD

Specifically AL / AX / EAX it depends on how much the specific width, STOS command bits D similarly affected (Direction Flag), when the D bit is 1, the value of EDI will be reduced, when the D bit is 0 the value of EDI will be added in front of the posts have talked about, MOVS instruction is also subject to the impact position D

Before writing something to the memory address, are used in DS :, here when used EDI, uniform use of ES, segment registers, not involved, for the time being the first so remember

　　MOV EAX,12345678

　　MOV EDI,18FF8C

　　DWORD PTR ES faro [EDI]

　　ES stosic WORD PTR [EDI]

　　HESE BYTE PTR ES: [EDI]

Pressing the F8, the instruction execution STOSD careful observations of EDI

It can be seen in the implementation of STOSD After instruction, the address number inside EAX value to EDI in writing represented, minus the value of EDI 4, look at STOSW, STOSB

It can be seen after you execute STOSW instruction, the lower 16 bits of EAX in the lower 16 bits of memory to EDI numbers represented in writing, minus the value of EDI 2, due to the continuous execution of our code, this step EDI value of the step value is decremented after the EDI 4, i.e. 0x18FF88

Let's look at STOSB command, and then press F8 to go down the code

Here it can be seen that, after subtracting the value of EDI 0x18FF86 a byte becomes 0x18FF85, then writing the value of AL in the EDI

We will change the Direction Flag value is 0, the following code is executed

　　MOV EAX,12345678

　　 MOV EDI, 12FFC4

　　HESE BYTE PTR ES: [EDI]

　　 ES stosic WORD PTR [EDI]

　　DWORD PTR ES faro [EDI]

After performing the first step, the second step the following code:

Do not forget that before the implementation of the D position to 0, I forgot to add the code of the time, when executed STOSB change back , then perform STOSB, carefully observe the following:

Through observation, we see the value of EDI plus 1, the value is written to the AL 0x18FF70, the Note: Only the lower 8 bits are written! ! !

Execution STOSW

We observed that, after the value from the EDI 0x18FF71 plus 2, becomes 0x18FF73, 0x18FF70 observation memory area can be seen, 0x18FF71,0x18FF72 written, the two values 56, 78

Instruction execution STOSD

EDI value from 0x18FF73 after 4 plus turns 0x18FF77, form if you do not understand why the reader is written in the form shown in the figure, I suggest that you put the memory split open, with a byte by byte to see and then draw a map, I believe you will understand that you can have 0x18FF73 starting point for the beginning, 4 bytes 4 bytes re-draw a map of their own, this time behind the memory number will be 0x18FF77,0x18FF7B, 0x18FF7F

REP instruction: Press count register (the ECX) is repeatedly executed the number of times specified in the instruction string

　　REP instruction count register according to the value specified in ECX times repeatedly executed instructions Note: The following code MOV ECX, 10, 10 represented by hexadecimal 10, 16 in decimal

　　 MOV EC, 10

　　 REP MOVSD

　　 REP STOSD