Final review compilation... Really working hard on the review...
The experiments assigned by the lab instructor, I feel grateful for them~
Let's talk about input/output, branching flow, basic instructions, and then discuss the questions we have done.
Input/Output
Converting ASCII code value of an integer to numerical value
- 0-9 30H-39H
Subtract 30H or AND with 1111(15) - A-F 41H-46H
Subtract 37H(55) - a-z 61H-66H
Subtract 57H(75)
Input a single hexadecimal integer
IN_1_HEX:
MOV AH, 01H
INT 21H ; The input value is stored in AL
CMP AL, '9' ; The input value is represented in memory as an ASCII code value
JBE IN
SUB AL, 07H
IN:
SUB AL, 30H
Input a two-digit hexadecimal integer
The maximum value represented by two-digit hexadecimal is FF
, which is 255, and it can be stored in eight bits.
IN_2_HEX:
CALL IN_1_HEX ; High-order hexadecimal -> AL
MOV AH, 10H
MUL AH
MOV AH, AL
CALL IN_1_HEX ; Low-order hexadecimal -> AL
ADD AL, AH
Input a single decimal integer
Remember to protect the previous data when using it, first PUSH then POP
IN_1_DEC:
MOV AH, 01H ; AL
INT 21H
SUB AL, 30H
Input a two-digit decimal integer
IN_2_DEC:
CALL IN_1_DEC
MOV AH, 10 ; In hexadecimal, this is 10H
MUL AH
MOV AH, AL
CALL IN_1_DEC
ADD AL, AH
Output a single hexadecimal integer
DISP_1_HEX:
CMP DL, 09H
JBE L1
ADD DL, 07H
L1:
ADD DL, 30H
MOV AH, 02H
INT 21H
Output a two-digit hexadecimal integer
DISP_2_HEX:
MOV AL, DL
MOV AH, 0
MOV DL, 10H
DIV DL ; Divisor is 8 bits, quotient is stored in AL, remainder in AH
MOV DL, AL
CALL DISP_1_HEX
MOV DL, AH
CALL DISP_1_HEX
Output a single decimal integer
DISP_1_DEC:
PUSH AX
ADD DL,30H
MOV AH,02H
INT 21H
POP AX
RET
Output a two-digit decimal integer
DISP_2_DEC: ; DL division by ten
PUSH AX
MOV AL,DL
MOV AH,0
MOV DL,10
DIV DL ; Divisor is 8 bits, quotient is stored in AL, remainder in AH
MOV DL,AL
CALL DISP_1_DEC
MOV DL,AH
CALL DISP_1_DEC
POP AX
RET
Output a multi-digit decimal integer with a sign bit
DISP: ; Output the number in AX in decimal
PUSHF
PUSH DX
PUSH AX
PUSH BX
PUSH CX
MOV CX, 0
MOV BX, 10 ; Divisor is 10
TEST AX, 8000H ; Check if the most significant bit of AX is 1 or 0
JE DISP1 ; If the result of the previous instruction is 0, jump
CALL FF ; Output the negative sign
NEG AX ; Negate +1
DISP1:
MOV DX, 0
DIV BX ; Divisor is 16-bit, quotient in AX, remainder in DX
PUSH DX
INC CX
OR AX, AX ; Check if quotient is 0
JNE DISP1 ; If not 0, continue division
DISP2:
POP DX
ADD DL, 30H ; Output as ASCII code
MOV AH, 02H
INT 21H
LOOP DISP2
POP CX
POP BX
POP AX
POP DX
POPF
RET
FF:
PUSH DX
PUSH AX
MOV DL, '-'
MOV AH, 02H
INT 21H
POP AX
POP DX
RET
Loop and Branching Structure
Comparison
The most important thing is comparison,
TEST
, AND
, CMP
, SUB
, and so on... can all be used for comparison.
TEST Instruction
Instruction format: TEST DST, SRC
The TEST
instruction can be used to check if a particular bit is 1, as it essentially performs an AND operation, i.e., DST & SRC
, without modifying the source and destination operands.
For example, the result of 00010000 & X
represents whether the fifth bit from the right in X is 1. If the result is 0, it means that the bit is 0, otherwise it is 1.
CMP Instruction
Instruction format: CMP DST, SRC
It can be used to compare the sizes of two numbers, essentially performing a subtraction operation, i.e., DST - SRC
, without modifying the source and destination operands.
Jump Instructions
Here, we determine the specific comparison of the two numbers being compared and decide which statement to execute.
In C-style languages, it would be like this:
if (a > b){
// Execute this if a > b
}else{
// Execute this if a <= b
}
In assembly language, it would be:
CMP AX, BX
JA AGB
# Execute this if AX <= BX
AGB:
# Execute this if AX > BX
The JA
above is a jump instruction. I won't go into detail about various instructions, refer to the book~
Here are a few important ones to remember~
- JE Two numbers are equal
- JNE Two numbers are not equal
- JA Unsigned, first number is greater than second number
This is how I remember it: A represents the first number among the two being compared, B represents the second number, soJA
means the first number is greater. - JG Signed comparison, first number is greater than second number
G stands for greater than - JB Unsigned, first number is less than second number
- JL Signed comparison, first number is less than second number
L stands for less than - JAE Unsigned, first number is greater than or equal to second number
- JGE Signed, first number is greater than or equal to second number
- JBE Unsigned, first number is less than or equal to second number
- JLE Signed, first number is less than or equal to second number
Binary to Hexadecimal Conversion
We all know that one hexadecimal digit can represent four binary digits, so to convert binary to hexadecimal, we need to convert four digits at a time.
The general process is as follows:
digraph g {
Shift left four binary digits
-> Take the low four digits
-> "Low four digits + 30H"
-> Output the low four digits;
Output the low four digits -> Shift left four binary digits [label="If there are more digits"]
}
Control Transfer Instructions
Refer to page 85 of the book
Questions
Input a twenty-digit signed hexadecimal number, sort it, and output the maximum, minimum, second maximum, and second minimum numbers in decimal
DATA SEGMENT
X DW 20 DUP(?)
NUM DW 6
DATA ENDS
S1 SEGMENT PARA STACK
BUF1 DW 20H DUP (0)
LEN1 EQU $-BUF1
S1 ENDS
CODE SEGMENT
ASSUME CS:CODE,DS:DATA,SS:S1
GO:
MOV AX,DATA
MOV DS,AX
MOV AX,S1
MOV SS,AX
MOV SP,LEN1
MOV CX,NUM
MOV SI,OFFSET X
L1:
CALL IN_2_HEX
MOV AH, AL
CALL IN_2_HEX
MOV [SI],AX
CC:
CALL SPACE
ADD SI,2
LOOP L1
CALL HR
MOV CX,NUM
MOV SI,OFFSET X
DEC CX
LOOP1:
PUSH CX
MOV BX,OFFSET X
LOOP2:
MOV AX,[BX]
CMP AX,[BX+2]
JGE CONTINUE ; Signed comparison
XCHG AX,[BX+2]
MOV [BX],AX
CONTINUE:
ADD BX,2
LOOP LOOP2
POP CX
LOOP LOOP1
MOV cx, NUM
MOV si,offset X
CALL HR
dispdec2:
MOV AX,[si]
call DISP
call SPACE
add si,2
loop dispdec2
CALL HR
MOV si,offset X
CALL HR
; Minimum number
DEC NUM
SHL NUM, 1
ADD SI,NUM
MOV AX,[SI]
call DISP
call SPACE
; Maximum number
MOV si,offset X
MOV AX,[si]
call DISP
call SPACE
; Second minimum number
ADD SI,NUM
MOV AX,[SI-2]
call DISP
call SPACE
; Second maximum number
MOV si,offset X
MOV AX,[SI+2]
call DISP
call HR
MOV AH,4CH
INT 21H
IN_2_HEX:
PUSHF
PUSH BX
MOV BH,AH
CALL IN_1_HEX ; AL high
MOV AH,10H
MUL AH ;
MOV AH,AL
CALL IN_1_HEX ; AL low
ADD AL,AH
MOV AH,BH
POP BX
POPF
RET
IN_1_HEX:
PUSHF
PUSH BX
MOV BH,AH
MOV AH,01H
INT 21H
cmp AL,'9'
JBE IN_B
SUB AL,07H
IN_B: ; 'A-F'
SUB AL,30H
MOV AH,BH
POP BX
POPF
RET
DISP:
PUSHF
PUSH DX
PUSH AX
PUSH BX
PUSH CX
MOV CX,0
MOV BX,10
test AX,8000H; Check if negative
JE DISP1
CALL FF
;AND AX,7FFFH
NEG AX
DISP1:
MOV DX,0
DIV BX ;AX, quotient; DX, remainder
PUSH DX
INC CX
OR AX,AX ; Check if 0
JNE DISP1
DISP2:
MOV AH,2
POP DX
ADD DL,30H
INT 21H
LOOP DISP2
POP CX
POP BX
POP AX
POP DX
POPF
RET
SPACE:
PUSH DX
PUSH AX
MOV DL,20H
MOV AH,02H
INT 21H
POP AX
POP DX
RET
HR:
PUSH AX
PUSH DX
MOV AH,02H
MOV DL,0AH
INT 21H
MOV DL,0DH
INT 21H
POP DX
POP AX
RET
FF:
PUSH DX
PUSH AX
MOV DL,'-'
MOV AH,02H
INT 21H
POP AX
POP DX
RET
CODE ENDS
END GO