CHAPTER 3. BOOT SECTOR PROGRAMMING (IN 16-BIT REAL
MODE) 25
;
; A simple boot sector program that demonstrates segment offsetting
;
mov ah, 0x0e ; int 10/ah = 0eh -> scrolling teletype BIOS routinemov al, [the_secret]
int 0x10 ; Does this print an X?
mov bx, 0x7c0 ; Can’t set ds directly , so set bx
mov ds, bx ; then copy bx to ds.
mov al, [the_secret]
int 0x10 ; Does this print an X?mov al, [es:the_secret] ; Tell the CPU to use the es (not ds) segment.
int 0x10 ; Does this print an X?mov bx, 0x7c0
mov es, bx
mov al, [es:the_secret]
int 0x10 ; Does this print an X?
jmp $ ; Jump forever.the_secret:
db "X"; Padding and magic BIOS number.
times 510-($-$$) db 0
dw 0xaa55Figure 3.7: Manipulating the data segment with thedsregister.
float above and another below it. Figure 3.8 shows the inside of a typical hard disk
drive, with the stack of platters and heads exposed. Note that the same idea applies to
floppy disk drives, which, instead of several stacked hard platters, usually have a single,
two-sided floppy disk medium.
The metalic coating of the platters give them the property that specific areas of their
surface can be magnetised or demagnetised by the head, effectively allowing any state
to be recorded permanently on them [?]. It is therefore important to be able to describe
the exact place on the disk’s surface where some state is to be read or written, and
so Cylinder-Head-Sector (CHS) addressing is used, which effectively is a 3D coordinate
system (see Figure 3.9):
- Cylinder: the cylinder describes the head’s discrete distance from the outer edge
of the platter and is so named since, when several platters are stacked up, you
can visualise that all of the heads select a cylinder through all of the platters - Head: the head describes which track (i.e. which specific platter surface within
the cylinder) we are interested in. - Sector: the circular track is divided into sectors, usually of capacity 512 bytes,
which can be referenced with a sector index.