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Assembler Final Cheat Sheet (DRAFT) by

This is a draft cheat sheet. It is a work in progress and is not finished yet.

maths

IMUL
Multiply ax by what ever is specified (for 32 bit store in DX:AX)
DIV
(16 bit) takes the operand and divides it by AX and stores it in AL with remainder in AH. for 32 bit it used DX:AX pair and leaves awn is AX and remainder in DX
MUL
Multiply ax by what ever is specified (all unsigned)
CWD
convers the word in AX to a double word in DX:AX

Convert num to bytes

ASCII
1 byte per char
unsigned
2 bytes for 5 chars
bcd
2 bytes for 5 chars
 

code examples

Write code that would find the sum 6+12+18...+300 and store it in var tot
MOV TOT, 0; MOV AX,6; LOOP: ADD TOT, AX; ADD AX, 6 CMP AX,300; JLE LOOP

write code that is assembler equiv: if(x<y){x++;}esle{y+= 2}
MOV AX, X; CMP AX,Y; JGW ELSE; ADD AX,1 JMP END; ELSE: ASS Y,2; END:

move 500 bytes of data TABLE1 to TABLE 2 using MOVSB
LEA SI, TABLE1; LEA DI, TABLE2; MOV CX, 500; CLD; LPTOP: MOVSB; LOOP LPTOP

MOVE 500 WORDS OF DATA FROM TABLE1 TO TABLE 2 USING INDEXING
MOV CX,500; MOV BX,0; LPTOP: MOV AX, TABLE1[BX]; MOV TABLE2[BX],AX; ADD BX, 2; LOOP LPTOP

count the number of blanks in the 1000 byte string of chars referanced by table 1 using scasb
MOV AX, SEG TABLE1; MOV ES, AX; MOV AL, ' '; LEA DI, TABLE1; COV CNT,0; CLD; LPTOP: SC ASB; JNE: SKIP; INC CNT; SKIP: LOOP LPTOP

binary

signed magnitude
Very left bit is 0 for + num and 1 for -num
twos compliment
flip the bits and add 1
27 excises
add the num to 128 then convert to binary
ones compliment
flip all bits
unsigned
all bits count but its a positive num
 

Bit shifting

RCR
rotate right last bite gets stored in carry and carry gets pushed to the first bite
SHL
Shift left into cf
TEST
The TEST operation sets the flags CF and OF to zero. The SF is set to the most signif­icant bit of the result of the AND. If the result is 0, the ZF is set to 1, otherwise set to 0. The parity flag is set to the bitwise XNOR of the least signif­icant byte of the result, 1 if the number of ones in that byte is even, 0 otherwise. The value of AF is undefined.
SAR
shift right into carry but keep the signed bit the same
CMC
invert CF
ROL
roatate left into the last bit and the carry flag
CLC
CF = 0
STC
CF = 1

adressing

tab[di]
indexed adressing [offset + ds *10 + DI]
[bx][di]
base indexi­ng[reg 1 + reg 2 + ds * 10]
[si]
register indire­ct[­ds*10 + reg1]
[bp]
base addres­sin­g[ss*10 + reg1]
 

Loads

LODSB
loads al wiht copy of DS:SI. IF DF = 0 then si++
LODSW
loads ax wiht copy of DS:SI. IF DF = 0 then si ++
STOSB
replace byte pointed to by ES:DI with a copy of AL and incs DI
STOSW
replace byte pointed to by ES:DI with a copy of AX and incs DI
CLD
clears DF
STD
set DF
MOVSW
replaces byte pointed to by ES:DI with word at DS:SI. Moves SI:DI by 2
MOVSB
copies the byte at [DS:SI] or [DS:ESI] to [ES:DI] or [ES:EDI]. It then increments or decrements (depending on the direction flag: increments if the flag is clear, decrements if it is set) SI and DI (or ESI and EDI).