16 bit Assembly

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Floppy Disk Parameters

Overview

Every disk holds its parameters in the Boot sector, if these details become corrupted, it will be difficult and often impossible to access the disk.

The parameeters are stored in the first 70 bytes of the Boot sector, starting from the 4th byte.
The first 3 bytes are a jump instruction pointing to the start of the bootup code.

There 2 ways to obtain disk parameter.

1. Request the parameters using a Bios call. This returns a limited number of values as shown below.
   ; Retrieves the drive parameters ; Call AH = 08h ; Pass DL = drive (bit 7 set for hard disk) ; ES:DI = 0000h:0000h to guard against BIOS bugs ; Returns: ; CF set on error, clear if successful ; AH = status ; AH = 00h ; BL = drive type ; CH = low eight bits of maximum cylinder number ; CL = maximum sector number (bits 5-0) ; high two bits of maximum cylinder number (bits 7-6) ; DH = maximum head number ; DL = number of drives ; ES:DI = drive parameter table (floppies only) SubGetFloppyParameters: PUSH ES MOV DI, 0x0000 PUSH EDI POP ES ; Load ES with DI = NULL MOV AX, 0x0800 MOV DX, 0x0000 INT 0x13 POP ES RET
2. Read the Parameters from the disk Boot sector by loading the Boot sector into memory. This can be done easily with the following routine.
   ; Loads the BOOT Sector into memory ; For Drive A: SubLoadFATBOOT: MOV SI, DataLoadingBOOTTXT CALL SubTTYString MOV CX, 0x0001 ; CX=Cylinder 1, Sector 1 MOV DX, 0x0000 ; DX Head 0 on Drive A MOV BX, 0x6000 ; Download to this address MOV AX, 0x0201 ; AX=Call to read 1 sector RET

Parameter Details in the Boot Sector

These are the parameters on my floppy disk. By reading the Boot sector to memory you can read and display the parameters. However the bottom 3 (Cylinders, Drive Type and Total Drives) are obtains from the BIOS interupt call.

The Boot sector will look similar to the hex dump below. The paremater block has been highlighted in yellow.

The values below are shown in the boot sequence in Hexidecimal. Values in brackets are the decimal values.

E9 43 00	Jump to start of boot code
50 45 52 2D 4F 53 2D 31	"PER-OS-1" manufacturers name
00 02	bytes per sector (512)
01	Sectors per Allocation (1)
01 00	Reserved Sectors (1)
02	Total FATS (2)
E0 00	Root Entries (224)
40 0B	Total Sectors (2880)
F0	Descriptor = 1.44Mb, 18 sectors, 2 heads
09 00	Sectors per FAT (9)
12 00	Sectors per Cylinder (18)
02 00	Total Heads (0 and 1)
00 00 00 00	Hidden Sectors
00 00 00 00	Total Sectors > 64k (mainly USB and hard drives)
00	Drive Number (0=A:)
00	Reserved
28	Extended Boot Signature (40), 0x28h for old floppy formats else 0x29h
30 30 30 31	Serial Number
50 45 52 50 45 54 55 41 4C 4F 53	"PERPETUALOS" Volume Label
46 41 54 31 32 20 20 20	FAT Type Name "FAT12"
00 00 00 00 00 00 00 00	Reserved

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Last Update 14/01/2023