1. Problem Analysis
We need to write a subroutine that displays a given byte of data in hexadecimal format at the center of the screen. One byte should be represented by two hexadecimal characters, corresponding to the high 4 bits and low 4 bits of the value.
Key Points:
- Separate the high 4 bits and low 4 bits of the byte
- Convert the 4-bit binary value (0-15) to the corresponding ASCII character
- Display these two characters at the specified position on the screen
2. Comparison of Solutions
1. Traditional Conditional Judgment Method
; Pseudocode example
cmp al, 0
je show_0
cmp al, 1
je show_1
...
cmp al, 15
je show_F
Disadvantage: The code is lengthy and inefficient, requiring 16 comparisons.
2. Lookup Table Method
Advantage: Concise and efficient, requiring only one lookup operation.
3. Detailed Implementation of the Lookup Table Method
1. Establishing the Conversion Table
hex_table db '0123456789ABCDEF' ; Hexadecimal character table
2. Complete Subroutine Implementation
; Function: Display the byte in hexadecimal form at the center of the screen
; Input: AL = byte to display
; Output: None
show_hex proc near
push ax
push bx
push cx
push ds
mov cx, cs ; Set data segment to code segment
mov ds, cx
mov ah, al ; Save original value
; Process high 4 bits
mov cl, 4
shr al, cl ; Shift right 4 bits to get high 4 bits
lea bx, hex_table
xlat ; AL = [BX + AL]
mov ch, al ; Save ASCII code of high bit
; Process low 4 bits
mov al, ah ; Restore original value
and al, 0Fh ; Get low 4 bits
xlat ; AL = [BX + AL]
; Display at the center of the screen (line 12, column 40)
mov bx, 0B800h ; Video memory segment address
mov es, bx
mov di, (12*80 + 40)*2 ; Calculate video memory position
mov es:[di], ch ; Display high bit
mov es:[di+2], al ; Display low bit
pop ds
pop cx
pop bx
pop ax
ret
hex_table db '0123456789ABCDEF' ; Character table
show_hex endp
4. Key Instruction Analysis
-
SHR: Logical right shift instruction used to separate high 4 bits
shr al, 4 ; Shift AL right by 4 bits, high 4 bits move to low 4 bits -
AND: Bitwise AND used to mask high 4 bits to get low 4 bits
and al, 0Fh ; Keep only low 4 bits -
XLAT: Table lookup conversion instruction
lea bx, hex_table ; BX points to the start of the table xlat ; AL = [BX + AL] -
Video Memory Operations:
mov es:[di], al ; Write ASCII character to video memory
5. Usage Example
assume cs:code
code segment
start:
mov al, 2Bh ; Value to display
call show_hex ; Call display subroutine
mov ax, 4C00h
int 21h
; Include the above show_hex subroutine
code ends
end start
6. Performance Optimization Techniques
-
Table Alignment: Placing the character table on a 256-byte boundary can speed up XLAT operations
org 100h align 256 hex_table db '0123456789ABCDEF' -
Register Reuse: Reduce the number of times registers are saved/restored
-
Loop Unrolling: If multiple bytes need to be displayed, loops can be unrolled
7. Extended Applications
Display Word (16-bit) Data:
show_word proc
push ax
mov al, ah ; Display high byte first
call show_hex
pop ax ; Then display low byte
call show_hex
ret
show_word endp
The lookup table method is an efficient technique for data conversion in assembly language, particularly suitable for scenarios with fixed mapping relationships, such as base conversion and encoding conversion. This method not only results in concise code but also has far superior execution efficiency compared to conditional judgment methods.