x86 compilation Notes

① several large registers as well as the usual effects.

Free 16-bit: AX (data segment), BX (offset address), CX (counter), DX (temporary data)

AH (break, carry), AL, bh/l,ch/l,dh/l

Pointer 16 bits: SP,BP

Index 16 bits: SI, DI (both as pointers to offset addresses)

Segment 16 bits: CS, DS, SS, ES

Directives & Signs: Not studied for the time being

[Limit] Offset address register: Bx,si,di

[Limit] counter register: Cx,loop statement detection =dec CX&JNZ

② Common Statements

MOV ax,01//assignment, register as left value

Add AX,BX//Do not carry addition, here carry refers to overflow problem

Since the maximum value of 16 bits in 8086 is 0xFFFF (65535), exceeding this range overflow, the result = True -65536* (carry +1), more than 16 bits of data, non-intelligent carry, need to look at the bytes after double byte, and *65536 restore calculation

So to record the overflow carry, and finally to restore the truth

The four-segment structure of ③ assembler program

SS (stack segment): usually one statement. Stack 64, set stack size 64KB

DS (data segment): Various variables are stored

Storage rule: small-end format. such as 16-bit data 35f3h, starting at offset position 0000, then 0000 of the 8-bit data f3,0001 is 35, the string is not the rule store.

CS (Code snippet): This is where the code is.

ES (additional segment): not yet studied

Several address issues

Logical address = Segment Address: Offset address (16 bits, upper and lower bounds 0/ffff), such as 2,500:0100

Physical Address = Segment Address shift left one (right 0) + offset address (20 bit)

④ Basic variables

EQU Constants

DB single byte, DW double byte, DD four bytes, DQ eight bytes, DT 10 bytes

DB can only be 8/16-bit registers, DW only 16-bit registers.

DD calculation, in 8086 need to cycle shift 2 times, DQ 4 times, DT 5 times

④ Basic Assembler Program

Double-byte summation

. MODEL SMALL. STACK -. DataArray DW23333,23333,23333,23333,23333SUM DW2DUP (0). Codemain PROC far mov AX, @DATA mov ds,ax mov si,offset ARRAY MOV CX, toMOV DX,0MOV BX,0For:add Dx,[si] ADD DI, GenevaADC BX,0DEC CX jnz for mov sum,dx mov SUM+2, BX MOV ah,4ch INT 21HMAIN endpend MAIN

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@ADC usage: When an ADC does addition, it secretly adds the previous accumulated carry value, clearing the CLC

@DW 2 is equivalent to a dd,dup for multivariable initialization

@MOV Ah,4ch&int 21H is the interrupt flag, indicating the end of the program

Double multi-byte loop accumulation

. MODEL SMALL. STACK -. Datacount EQU GenevaDATA1 DD12345678DATA2 DD23456789SUM DD0. Codemain PROC far mov AX, @DATA mov ds,ax mov cx,count mov si,offset DATA1 mov di,offset DATA2 mov bx,off SET SUM For:mov Ax,[si] ADC Ax,[di] MOV [Bx],ax ADD SI, GenevaADD DI, GenevaADD BX, GenevaLOOP for MOV ah,4ch INT 21HMAIN endpend MAIN

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@LOOP usage: self-reducing circulator with CX binding

The @ carry can be transferred to the next calculation without having to be stored separately and multibyte addition.

x86 compilation Notes