8.突破512字节的限制（下）

Posted 2021-08-30 阿弥陀佛.a

这一篇的重点是通过FAT表项将文件内容加载进入内存

在根目录区每一项占32字节，对我们有用的只有这32字节，只拷贝这32字节有利于不浪费内存：

原内存与目标内存有重叠的情况，那么拷贝的方向就至关重要！！！
图中内存向下增长。拷贝内容起始地址比目标起始地址低时，应该从尾部向头部进行拷贝。

si大于di就跳转至btoe

汇编代码如下：

org 0x7c00

jmp short start
nop

define:
    BaseOfStack      equ 0x7c00
    BaseOfLoader     equ 0x9000//目标程序加载到这个地址
    RootEntryOffset  equ 19
    RootEntryLength  equ 14
    EntryItemLength  equ 32
    FatEntryOffset   equ 1
    FatEntryLength   equ 9

header:
    BS_OEMName     db "D.T.Soft"
    BPB_BytsPerSec dw 512
    BPB_SecPerClus db 1
    BPB_RsvdSecCnt dw 1
    BPB_NumFATs    db 2
    BPB_RootEntCnt dw 224
    BPB_TotSec16   dw 2880
    BPB_Media      db 0xF0
    BPB_FATSz16    dw 9
    BPB_SecPerTrk  dw 18
    BPB_NumHeads   dw 2
    BPB_HiddSec    dd 0
    BPB_TotSec32   dd 0
    BS_DrvNum      db 0
    BS_Reserved1   db 0
    BS_BootSig     db 0x29
    BS_VolID       dd 0
    BS_VolLab      db "D.T.OS-0.01"
    BS_FileSysType db "FAT12   "

start:
    mov ax, cs
	mov ss, ax
	mov ds, ax
	mov es, ax
	mov sp, BaseOfStack
	
	mov ax, RootEntryOffset
	mov cx, RootEntryLength
	mov bx, Buf
	
	call ReadSector
	
	mov si, Target
	mov cx, TarLen
	mov dx, 0
	
	call FindEntry
	
	cmp dx, 0
	jz output
	
	mov si, bx
	mov di, EntryItem
	mov cx, EntryItemLength
	
	call MemCpy
	
	mov ax, FatEntryLength
	mov cx, [BPB_BytsPerSec]
	mul cx
	mov bx, BaseOfLoader
	sub bx, ax
	
	mov ax, FatEntryOffset
	mov cx, FatEntryLength
	
	call ReadSector
	
	mov cx, [EntryItem + 0x1A]
	
	call FatVec
	
	jmp last
	
output:	
    mov bp, MsgStr
    mov cx, MsgLen
	call Print
	
last:
    hlt
	jmp last	


; cx --> index//表示Fat表项的下标
; bx --> fat table address//Fat表在内存中的起始地址
;
; return:
;     dx --> fat[index]
FatVec:
    mov ax, cx
    mov cl, 2
    div cl
    
    push ax
    
    mov ah, 0
    mov cx, 3
    mul cx
    mov cx, ax
    
    pop ax
    
    cmp ah, 0
    jz even
    jmp odd

even:    ; FatVec[j] = ( (Fat[i+1] & 0x0F) << 8 ) | Fat[i];
    mov dx, cx
    add dx, 1
    add dx, bx
    mov bp, dx
    mov dl, byte [bp]
    and dl, 0x0F
    shl dx, 8
    add cx, bx
    mov bp, cx
    or  dl, byte [bp]
    jmp return
    
odd:     ; FatVec[j+1] = (Fat[i+2] << 4) | ( (Fat[i+1] >> 4) & 0x0F );
    mov dx, cx
    add dx, 2
    add dx, bx
    mov bp, dx
    mov dl, byte [bp]
    mov dh, 0
    shl dx, 4
    add cx, 1
    add cx, bx
    mov bp, cx
    mov cl, byte [bp]
    shr cl, 4
    and cl, 0x0F
    mov ch, 0
    or  dx, cx

return: 
    ret

; ds:si --> source
; es:di --> destination
; cx    --> length
MemCpy:
    push si
    push di
    push cx
    push ax
    
    cmp si, di
    ja btoe
    
    add si, cx//从后向前拷贝的准备
    add di, cx
    dec si
    dec di
    
    jmp etob
    
btoe://从前往后拷贝
    cmp cx, 0//一直循环到拷贝cx个字节
    jz done
    mov al, [si]
    mov byte [di], al
    inc si
    inc di
    dec cx
    jmp btoe
    
etob: //从后往前拷贝
    cmp cx, 0
    jz done
    mov al, [si]
    mov byte [di], al
    dec si
    dec di
    dec cx
    jmp etob

done:   
    pop ax
    pop cx
    pop di
    pop si
    ret

; es:bx --> root entry offset address
; ds:si --> target string
; cx    --> target length
;
; return:
;     (dx !=0 ) ? exist : noexist
;        exist --> bx is the target entry
FindEntry:
    push di
    push bp
    push cx
    
    mov dx, [BPB_RootEntCnt]
    mov bp, sp
    
find:
    cmp dx, 0
    jz noexist
    mov di, bx
    mov cx, [bp]
    call MemCmp
    cmp cx, 0
    jz exist
    add bx, 32
    dec dx
    jmp find

exist:
noexist: 
    pop cx
    pop bp
    pop di
       
    ret

; ds:si --> source
; es:di --> destination
; cx    --> length
;
; return:
;        (cx == 0) ? equal : noequal
MemCmp:
    push si
    push di
    push ax
    
compare:
    cmp cx, 0
    jz equal
    mov al, [si]
    cmp al, byte [di]
    jz goon
    jmp noequal
goon:
    inc si
    inc di
    dec cx
    jmp compare
    
equal: 
noequal:   
    pop ax
    pop di
    pop si
    
    ret

; es:bp --> string address
; cx    --> string length
Print:
    mov dx, 0//设置为0代表将目标字符串打印到屏幕左上角
    mov ax, 0x1301
	mov bx, 0x0007
	int 0x10
    ret

; no parameter
ResetFloppy:
    push ax
    push dx
    
    mov ah, 0x00
    mov dl, [BS_DrvNum]
    int 0x13
    
    pop dx
    pop ax
    
    ret

; ax    --> logic sector number
; cx    --> number of sector
; es:bx --> target address
ReadSector:
    push bx
    push cx
    push dx
    push ax
    
    call ResetFloppy
    
    push bx
    push cx
    
    mov bl, [BPB_SecPerTrk]
    div bl
    mov cl, ah
    add cl, 1
    mov ch, al
    shr ch, 1
    mov dh, al
    and dh, 1
    mov dl, [BS_DrvNum]
    
    pop ax
    pop bx
    
    mov ah, 0x02

read:    
    int 0x13
    jc read
    
    pop ax
    pop dx
    pop cx
    pop bx
    
    ret

MsgStr db  "No LOADER ..."	
MsgLen equ ($-MsgStr)
Target db  "LOADER     "
TarLen equ ($-Target)
EntryItem times EntryItemLength db 0x00
Buf:
	times 510-($-$$) db 0x00
	db 0x55, 0xaa