INT 13 Extension Read in C - c

Question

i can use extended read functions of bios int 13h well from assembly,
with the below code
; *************************************************************************
; Setup DISK ADDRESS PACKET
; *************************************************************************
jmp strtRead
DAPACK :
db 010h ; Packet Size
db 0 ; Always 0
blkcnt:
dw 1 ; Sectors Count
db_add :
dw 07e00h ; Transfer Offset
dw 0 ; Transfer Segment
d_lba :
dd 1 ; Starting LBA(0 - n)
dd 0 ; Bios 48 bit LBA
; *************************************************************************
; Start Reading Sectors using INT13 Func 42
; *************************************************************************
strtRead:
mov si, OFFSET DAPACK; Load DPACK offset to SI
mov ah, 042h ; Function 42h
mov dl, 080h ; Drive ID
int 013h; Call INT13h
i want to convert this to be a c callable function but i have no idea about how to transfer the parameters from c to asm like drive id , sectors count, buffer segment:offset .... etc.
i am using msvc and masm and working with nothing except bios functions.
can anyone help ?!!
update :
i have tried the below function but always nothing loaded into the buffer ??
void read_sector()
{
static unsigned char currentMBR[512] = { 0 };
struct disk_packet //needed for int13 42h
{
byte size_pack; //size of packet must be 16 or 16+
byte reserved1; //reserved
byte no_of_blocks; //nof blocks for transfer
byte reserved2; //reserved
word offset; //offset address
word segment; //segment address
dword lba1;
dword lba2;
} disk_pack;
disk_pack.size_pack = 16; //set size to 16
disk_pack.no_of_blocks = 1; //1 block ie read one sector
disk_pack.reserved1 = 0; //reserved word
disk_pack.reserved2 = 0; //reserved word
disk_pack.segment = 0; //segment of buffer
disk_pack.offset = (word)&currentMBR[0]; //offset of buffer
disk_pack.lba1 = 0; //lba first 32 bits
disk_pack.lba2 = 0; //last 32 bit address
_asm
{
mov dl, 080h;
mov[disk_pack.segment], ds;
mov si, disk_pack;
mov ah, 42h;
int 13h
; jc NoError; //No error, ignore error code
; mov bError, ah; // Error, get the error code
NoError:
}
}

Answer 1

Sorry to post this as "answer"; I want to post this as "comment" but it is too long...
Different compilers have a different syntax of inline assembly. This means that the correct syntax of the following lines:
mov[disk_pack.segment], ds;
mov si, disk_pack;
... depends on the compiler used. Unfortunately I do not use 16-bit C compilers so I cannot help you in this point.
The next thing I see in your program is the following one:
disk_pack.segment = 0; //segment of buffer
disk_pack.offset = (word)&currentMBR[0]; //offset of buffer
With a 99% chance this will lead to a problem. Instead I would to the following:
struct disk_packet //needed for int13 42h
{
byte size_pack;
byte reserved;
word no_of_blocks; // note that this is 16-bit!
void far *data; // <- This line is the main change!
dword lba1;
dword lba2;
} disk_pack;
...
disk_pack.size_pack = 16;
disk_pack.no_of_blocks = 1;
disk_pack.reserved = 0;
disk_pack.data = &currentMBR[0]; // also note the change here
disk_pack.lba1 = 0;
disk_pack.lba2 = 0;
...
Note that some compilers name the keyword "_far" or "__far" instead of "far".
A third problem is that some (buggy) BIOSes require ES to be equal to the segment value from the disk_pack and a fourth one is that many compilers require the inline assembly code not to modify any registers (AX, CX and DX is normally OK).
These two could be solved the following way:
push ds;
push es;
push si;
mov dl, 080h;
// TODO here: Set ds:si to disk_pack in a compiler-specific way
mov es,[si+6];
mov ah, 42h;
int 13h;
...
pop si;
pop es;
pop ds;
In my opinion the "#pragma pack" should not be neccessary because all elements in the structure are propperly aligned.