Slightly more about loading self-made OS - write a bootloader
Not so long ago, I decided to study IA-32 architecture a little better. And what is best for memorization? Of course practice. But programming in the OS, we are unlikely to get the lowest level of access to hardware without interference. Therefore, for these purposes, we will write our own similarity of the operating system. That is, simply speaking, we will execute our code immediately after loading the BIOS.
The first problem that anyone who wants to program at a low level will face is how to load your code?
Usually in the BIOS there is a list of devices from which it is trying to boot, going through one by one. This list usually consists of a drive, a CD drive, and a hard disk. Booting from a floppy disk and a CD is almost the same — for compatibility, a floppy disk image is placed in the boot area of the disk, which is then copied into memory and acts as a virtual drive. And since booting from a floppy is the easiest, and we will use it.
After the BIOS detects all the devices and performs everything necessary for itself, it loads the first sector of the diskette into memory at address 0000: 7C00 and transfers control there. This is where we encounter the first problem - the size of the sector on a floppy disk is only 512 bytes in size, and we have to meet these limits in order to load the rest of the code. If this still seems to you a lot, I will say that for compatibility, some 60 more of them are spent on service purposes. Of course, you can throw them out, but then the diskette may not be visible on the systems, and copying files to it will be difficult.
To simplify, add this data later. At once I will make a reservation that all the above code will be given in the syntax of FASM.
So let's start with the simplest, getting control and text output.
If you think that we will finish on this, as in hundreds of other examples, then I will disappoint you, and maybe I will please you - our downloader will search the disk for the file and download it.
To begin, I will tell you about the layout of a floppy under FAT12.
The first sector is reserved for service data - a BIOS parameter block (BPB), as well as a boot code.
BPB in our case will look like this:
')
For further work with diskette and files, we will write several auxiliary procedures.
The first one is reading one sector at an absolute address, i.e. the sector will be set not by a set of parameters head, cluster, sector, but by one ordinal number.
For those who are not familiar with FASM I will explain about jump tags.
@@ is a universal label, it can occur any time in the code;
@b - jump to the first label @@ up the code (back);
@f - jump to the first label @@ further along the code (forward);
The next procedure is an add-on over this one, and it will read several sectors in a row. You can complain, but where are the checks? I do not check for correctness of sector numbers to save space.
As you can see, there is nothing difficult yet, but the functionality is added.
Now actually tell you what we need to read the file. To begin with we consider in memory the entire FAT table and root directory.
The entire table FAT12 consists of records of 12 bits (!), Which are combined into chains. The numbers indicate the absolute address of the sector. We read until the number is 0xFFF - this is the end of the chain.
Those. if the file is 513 bytes, then 2 sectors will be allocated for it, even though the second one will be occupied by one byte only.
Now as for the table of the main directory - it consists of 32-byte records, which contains all the data about the file.
Here is its format:
One feature - when deleting files, the records themselves are not deleted, but only the first byte of the name is replaced with the character 0xE5.
Files with zero length can not be - because thus folders are designated, and at offset + 1Ah the number of the first entry nested in the file is recorded.
The first two of which are. and .., which respectively indicate the first entry in the current directory, and the parent.
Let's write two more very small procedures - which will read both tables into memory.
Now, to read into the memory of the file, it remains to collect all that we have written, i.e. find an entry about it in the root directory, and count the sectors from FAT into memory.
Actually, this operation took the most time and code.
The first problem that anyone who wants to program at a low level will face is how to load your code?
Introduction
Usually in the BIOS there is a list of devices from which it is trying to boot, going through one by one. This list usually consists of a drive, a CD drive, and a hard disk. Booting from a floppy disk and a CD is almost the same — for compatibility, a floppy disk image is placed in the boot area of the disk, which is then copied into memory and acts as a virtual drive. And since booting from a floppy is the easiest, and we will use it.
After the BIOS detects all the devices and performs everything necessary for itself, it loads the first sector of the diskette into memory at address 0000: 7C00 and transfers control there. This is where we encounter the first problem - the size of the sector on a floppy disk is only 512 bytes in size, and we have to meet these limits in order to load the rest of the code. If this still seems to you a lot, I will say that for compatibility, some 60 more of them are spent on service purposes. Of course, you can throw them out, but then the diskette may not be visible on the systems, and copying files to it will be difficult.
To simplify, add this data later. At once I will make a reservation that all the above code will be given in the syntax of FASM.
So let's start with the simplest, getting control and text output.
Use16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaaUse16 org 0x7C00 start : cli ; mov ax , cs ; mov ds , ax mov es , ax mov ss , ax mov sp , 0x7C00 ; .. , mov ax , 0xB800 mov gs , ax ; mov si , msg call k_puts hlt ; jmp $ ; k_puts : lodsb test al , al jz . end_str mov ah , 0x0E mov bl , 0x07 ; int 0x10 jmp k_puts . end_str ret msg db 'Hello world' , 0x0d , 0x0a , 0 times 510 - ( $ - $$ ) db 0 db 0x55 , 0xaa
If you think that we will finish on this, as in hundreds of other examples, then I will disappoint you, and maybe I will please you - our downloader will search the disk for the file and download it.
To begin, I will tell you about the layout of a floppy under FAT12.
The first sector is reserved for service data - a BIOS parameter block (BPB), as well as a boot code.
BIOS Parameter Block
BPB in our case will look like this:
jmp start; jump to our code db 0 BS_OEMName db 'MicLib'; any text BPB_BytsPerSec dw 0x200; byte in the sector BPB_SecPerClus db 1; sectors in a cluster BPB_RsvdSecCnt dw 1; number of reserved sectors BPB_NumFATs db 2; number of FAT tables BPB_RootEntCnt dw 0x00E0; number of entries in the root tree BPB_TotSec16 dw 0x0B40 BPB_Media db 0xF0 BPB_FATSz16 dw 9; FAT size in sectors BPB_SecPerTrk dw 0x12; sectors on track BPB_NumHeads dw 2; number of reading heads BPB_HiddSec dd 0 BPB_TotSec32 dd 0
')
For further work with diskette and files, we will write several auxiliary procedures.
The first one is reading one sector at an absolute address, i.e. the sector will be set not by a set of parameters head, cluster, sector, but by one ordinal number.
- ;
- ; The procedure for reading the sector of the floppy by absolute number
- ;
- ; Entrance:
- ; dx - absolute sector number
- ; si - buffer address
- ;
- k_read_sector :
- ; S = N mod 18 + 1
- ; T = N / 18
- ; H = T mod 2
- ; C = T / 2
- pusha
- mov ax , dx
- mov cx , [ BPB_SecPerTrk ]
- mov bx si
- xor dx , dx ; Starting from here, we recalculate using the formulas
- div cx
- mov ch , al
- shr ch , 1
- mov cl , dl
- inc cx
- mov dh , al
- and dh , 1
- mov ax , 0x0201
- xor dl , dl
- int 0x13
- jnc @f ; If the C flag is set, an error has occurred.
- mov si , msgErrorRead
- call k_puts ; Let us know
- @@ :
- popa
- ret
For those who are not familiar with FASM I will explain about jump tags.
@@ is a universal label, it can occur any time in the code;
@b - jump to the first label @@ up the code (back);
@f - jump to the first label @@ further along the code (forward);
The next procedure is an add-on over this one, and it will read several sectors in a row. You can complain, but where are the checks? I do not check for correctness of sector numbers to save space.
- ;
- ; Procedure for sequential reading of multiple sectors
- ;
- ; Entrance:
- ; dx - starting sector
- ; cx - how many sectors to read in a row
- ; si - memory address where to read
- ;
- k_read_sectors :
- push dx
- push cx
- @@ :
- call k_read_sector
- inc dx
- add si , [ BPB_BytsPerSec ]
- dec cx
- jnz @b ; We read yet not 0
- pop cx
- pop dx
- ret
As you can see, there is nothing difficult yet, but the functionality is added.
Now actually tell you what we need to read the file. To begin with we consider in memory the entire FAT table and root directory.
FAT table and root directory
The entire table FAT12 consists of records of 12 bits (!), Which are combined into chains. The numbers indicate the absolute address of the sector. We read until the number is 0xFFF - this is the end of the chain.
Those. if the file is 513 bytes, then 2 sectors will be allocated for it, even though the second one will be occupied by one byte only.
Now as for the table of the main directory - it consists of 32-byte records, which contains all the data about the file.
Here is its format:
+0 11 File name in the format 'IIIIIIIIRRRR' The file name is 8 characters long, if shorter - filled with spaces. There is no separator point. 3byte extension + 0Bh 1 File Attributes: 01h - Read Only 02h - Hidden 04h - System 08h - Volume Tag 10h - Directory 20h - Archive + 0Ch 10 Reserved + 16h 2 Time to create or modify in filetime format + 18h 2 Date of creation or modification in filetime format + 1Ah 2 Number of the first entry in the chain of FAT + 1Ch 4 Size
One feature - when deleting files, the records themselves are not deleted, but only the first byte of the name is replaced with the character 0xE5.
Files with zero length can not be - because thus folders are designated, and at offset + 1Ah the number of the first entry nested in the file is recorded.
The first two of which are. and .., which respectively indicate the first entry in the current directory, and the parent.
Let's write two more very small procedures - which will read both tables into memory.
- ;
- ; The procedure reads the FAT table into memory.
- ;
- k_read_fat :
- mov dx , 1 ; It is located right behind the boot sector.
- mov cx , [ BPB_FATSz16 ] ; 9 sectors
- mov si , FAT
- call k_read_sectors
- ret
- ;
- ; The procedure reads the root directory into memory.
- ;
- k_read_root_dir :
- mov dx , 19 ; 1 + 9 * 2
- mov cx , 15
- mov si , ROOT
- call k_read_sectors
- ret
Reading file
Now, to read into the memory of the file, it remains to collect all that we have written, i.e. find an entry about it in the root directory, and count the sectors from FAT into memory.
Actually, this operation took the most time and code.
- ;
- ; The procedure reads a file from a floppy disk into memory.
- ;
- ; Entrance:
- ; di - buffer address
- ; si - the file name is strictly in the format NNNNNNNNEEE
- ; Output:
- ; ax - 0 if the file is not found, 1 - found
- ;
- k_read_file :
- push di
- mov di , root
- mov cx , 0xE0 ; BPB_RootEntCnt
- . next_item :
- mov al , byte [ di ]
- cmp al , 0xE5 ; Remote file label
- je . space_item
- cmp al , 0 ; Blank entry
- je . space_item
- push di
- push si
- push cx
- mov cx , 11 ; 8 + 3
- repe cmpsb ; Compare the file name with the desired
- cmp cx , 0
- pop cx
- pop si
- pop di
- je . read_file ; break
- . space_item :
- add di , 32 ; record length
- loop next_item
- xor ax , ax
- ; jmp .end_of_file
- ret
- . read_file :
- pop si
- mov bp , word [ di + 0x1A ] ; Initial cell number FAT
- mov bx , word [ di + 0x1C ] ; File size
- . read_next_claster :
- pusha
- mov dx , bp
- sub dx , 3
- add dx , 0x22
- call k_read_sector
- popa
- cmp di , 0xFFF
- je . end_of_file
- mov di , bp
- mov ax , bp ; save for parity check
- mov bx , bp ; save in case there will be 0xFFF
- imul di </ f
Source: https://habr.com/ru/post/107030/
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