245 lines
8.0 KiB
NASM
245 lines
8.0 KiB
NASM
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;;
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;; Copyright (c) Microsoft Corporation. All rights reserved.
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;;
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;;;;;;;;;;;;;;;;;;;;
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; Concerns
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; 1 - there is no error checking on the int13 calls
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; 2 - we assume that the block size is 2048 bytes
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; 3 - this cannot handle large root directories (>64KB)
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;;;;;;;;;;;;;;;;;;;;
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; Constants
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BootSecOrigin EQU 07c00h ; the BIOS puts the boot sector at 07c0h:0000 == 0000:7c00h
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StackOffset EQU -12 ; we will put the stack a small bit below it (we hardly use the stack, so it is safe...)
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;;;;;;;;;;;;;;;;;;;;
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; Directives
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.model tiny
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.686p
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;;;;;;;;;;;;;;;;;;;;
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; Begin Code segment
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_TEXT SEGMENT use16 ; 16-bit code segment
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.code
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ORG 0h ; ETFS puts us at 07c0h:0000
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start:
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Step1: ; set stack and data segments
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mov cx, cs
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mov ss, cx
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mov sp, BootSecOrigin + StackOffset
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mov es, cx
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mov ds, cx
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mov bp, sp
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Step2: ; Save the boot drive (dl holds it on boot)
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mov [CDDrive], dl
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Step3: ; Clear the Screen
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mov ax, 02h
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int 010h
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Step4: ; Load the PVD to get the Logical Block Size
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mov eax, 10h ; the PVD is in the 16th block
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mov bx, 2000h
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mov es, bx ; transfer address = 2000:0000
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mov cx, 1
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call ReadDisk
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mov ax, es:128 ; block size is at offset 128
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mov [BlockSize], ax
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Step5: ; Find the Joliet SVD, and then find the Root Directory Information
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mov eax, 10h ; start with the PVD, even though it will fail
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GetNextVD:
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push eax
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mov cx, 1
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call ReadDisk
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mov si, OFFSET SVDesc ; [ds:si] points to the desired first 6 bytes of this VD
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xor di, di ; [es:di] points to the start of what we just read
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mov cx, 6
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repe cmpsb
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je FoundSVD
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mov al, es:0000h
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cmp al, 0FFh ; is this the last Volume Descriptor?
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je SVDError
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pop eax
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inc eax
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jmp GetNextVD ; try another VD
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FoundSVD: ; need to make sure this is a Joliet SVD - we need 025h, 02Fh, 045h in [88,89,90]
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mov si, OFFSET JolietSig ; [ds:si] points to the Joliet Signature
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mov di, 88 ; [es:di] points to the escape sequence field of the current SVD
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mov cx, 3
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repe cmpsb
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je FoundJoliet
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pop eax
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inc eax
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jmp GetNextVD
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FoundJoliet:
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mov eax, es:158 ; now get the rootstart and rootsize fields
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mov [RootStart], eax
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mov eax, es:166
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mov [RootSize], eax
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Step6: ; Load the Root Directory (SVD), and search it for SINGLDR
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movzx ebx, [BlockSize]
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div ebx ; eax has # blocks in root directory. Round up if necessary:
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cmp edx, 0
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je ReadyToLoad
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add eax, 1
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ReadyToLoad: ; we're going to assume that the root directory will not be bigger than 64K
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mov ecx, eax
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mov eax, [RootStart]
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call ReadDisk
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xor ebx, ebx ; bx will hold the start of the current entry
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CheckEntry:
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mov di, bx
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add di, 25 ; let's check the file flags - should be 00
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mov al, es:[di]
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cmp al, 0
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jne PrepNextEntry
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; file flags are good. now check the file identifier:
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mov si, OFFSET Stage2FileSize
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xor cx, cx
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mov cl, ds:[si] ; first byte is file name length
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add cx, 2 ; add two because we check the length byte of the directory entry and the padding byte, too
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add di, 7 ; now es:di points to the file length/name field, and ds:si has our desired content
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repe cmpsb
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je FoundEntry
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PrepNextEntry:
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xor cx, cx ; increment bx by adding the byte value in es:[bx]
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mov cl, es:[bx] ; if es:[bx]==0 and ebx!= [RootSize], then we are in a padding zone
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cmp cx, 0 ; designed to prevent a directory entry from spilling over a block.
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jne LoadNext ; Should this be the case, we will increment bx until es:[bx] is not null
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inc bx
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jmp PrepNextEntry
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LoadNext:
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add bx, cx
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cmp ebx, [RootSize]
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jl CheckEntry
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jmp FileNotFoundError
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FoundEntry:
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mov eax, es:[bx+2]
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mov [FileStart], eax
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mov eax, es:[bx+10]
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mov [FileSize], eax
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Step7: ; Load the file to 57c0:0000
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mov cx, 057c0h
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mov es, cx
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movzx ebx, [BlockSize]
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div ebx ; eax has # blocks in root directory
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cmp edx, 0 ; on carry, there will be one more block
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je ReadyToLoadFile
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add eax, 1
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ReadyToLoadFile:
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mov ecx, eax
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mov eax, [FileStart]
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call ReadDisk
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Step8: ; Now we need to set up the stack for SINGLDR and do a jump
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xor cx, cx ; Always point the stack to 0000:7c00h - 12
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mov ss, cx
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mov sp, BootSecOrigin + StackOffset
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movzx edx, [CDDrive]
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push edx ; SINGLDR will need to know the boot drive #
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pushd 04344h ; CD boot signature
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pushw offset infloop ; return address = "infloop", which is the infinite loop
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push cs
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db 0EAh ; emit a long jump to 5000:7c00
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dd 50007c00h
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;;;;;;;;;;;;;;;;;;;;
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; ReadDisk
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;
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; Inputs: eax = Block Number
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; cx = number of blocks to read (warning: cx > 32 will cause overflow)
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; es = destination segment
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; Assumptions: 1 - assumes request will not cause overflow of es:00 (limit on # sectors)
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; 2 - assumes int13 extensions available
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ReadDisk PROC NEAR
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pushad
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mov dl, [CDDrive] ; set the drive
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pushd 00
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push eax ; push 64-bit block number (top half always null)
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push es
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pushw 00h ; push transfer address
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push cx ; # sectors
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pushw 0010h ; this request packet is 16 bytes
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mov ah,42h ; extended read
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mov si,sp ; ds:si = address of params
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int 13h ; perform the read
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add sp, 10h ; clean the stack and return
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popad
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ret
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ReadDisk ENDP
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;;;;;;;;;;;;;;;;;;;;
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; Error Routines (these are jump points that never return)
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SVDError:
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mov al, "S"
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jmp PrintError
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FileNotFoundError:
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mov al, "F"
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PrintError:
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mov bx, 07h ; normal attribute
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mov ah, 0eh ; default print 1 char
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mov cx, 6
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mov si, offset ErrorMsg
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int 10h
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printnext:
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lodsb
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or al, al
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jz infloop
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int 10h
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jmp printnext
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infloop:
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jmp infloop
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;;;;;;;;;;;;;;;;;;;;
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; Global Vars
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RootStart DD 0
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RootSize DD 0
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CDDrive DB 0
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BlockSize DW 0
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FileStart DD 0
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FileSize DD 0
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;;;;;;;;;;;;;;;;;;;;
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; String Constants
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SVDesc DB 02h, "CD001"
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JolietSig DB 25h, 2fh, 45h ; this is the value of the escape sequence for a Joliet CD
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; we'll use it as the signature...
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Stage2FileSize DB OFFSET Stage2FilePad - OFFSET Stage2File
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Stage2File DB 0,"S",0,"i",0,"n",0,"g",0,"l",0,"d",0,"r" ; in unicode, this is how our filename will appear
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Stage2FilePad DB 0
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ErrorMsg DB " Error", 0
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;;;;;;;;;;;;;;;;;;;;
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; Boot Sector Signature
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ORG 510
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DW 0AA55h
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end start
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