BIOS for the IBM PC if you want it.
The boot sector does three things
1. It lets DOS know the disk is indeed an MS or PCDOS formatted disk.
2. It provides a table (BIOS Parameter Block) of important values DOS needs to
know such as size of disk's sectors, clusters, and directory.
3. It runs a bootstrap program that looks for the main operating system files
and launches them.
The two FATs (File Allocation Tables) keep tabs on every cluster on your disk
1. DOS stores information by looking at the FAT to see where the first available
unused cluster is on the disk and puts data there.
2. 2. DOS retrieves data when it consults the FAT to see which cluster holds
information.
Root Directory is the main directory because it's the beginning of a tree
structure (hierarchical system) with subdirectories branching off it.
1. The Root Directory maintains the name, size, creation (or last modification)
time and date and the initial cluster location for a specific number of files
2. Directory maintains information about each file called an attribute.
Each directory takes up 32 (hex 20) bytes. The attribute is kept in the 12th
byte (byte #11 or hex 0B)
BIOS variables
0040:0000 - BIOS stores base I/O addresses of Com1 - Com4 in these eight bytes
0040:0008 - Table of I/O addresses for LPT1-4
0040:0017 Byte that holds status of keyboard's shift keys. This is the same byte
that INT 16h, function 12h return (If you can change bits in this byte you can
fool BIOS setting CAPS LOCK on ex., has the same effect as pressing CAPS LOCK
key on keyboard
0040:0018 This byte holds more information about the keyboard's state
In general, it is best not to change these bits. Except bit 3, the pause bit,
pressing PAUSE or CONTROL-NUM-LOCK key causes BIOS to stop most programs from
running. Interrupts still run if a program can't tolerate a pause it can set up
a timer interrupt routine and rest this bit on each tick. That way the program
can't be paused for more than 1/18th of a second at a time.
0040:001A These 2 bytes point to the head of BIOS keyboard buffer
0040:001C These 2 bytes point to the tail of the keyboard buffer
0040:001E This is the default keyboard buffer (16 two-byte words)
0040:0049 This byte stores current video mode. The notation is the same as INT
10h,function 0
0040:004A These 2 bytes indicate the number of characters per row the current
video mode allows
0040:004C This word holds the number of bytes required to store one screenful of
data
0040:004E This word stores the offset for the current video page
0040:0050 This table of 8 two-byte words that contain the current cursor
position. For each page the first byte is the column, the second byte is
the row
0040:0060 This byte holds the ending scanline for the cursor
0040:0061 This byte holds the starting scanline for the cursor
0040:0062 The BIOS stores the current video page number in this byte
0040:006C These four bytes contain the number of time ticks since midnight. This
is the same value returned by INT 1A function 00h
0040:0070 This byte contains the midnight flag used by INT 1Ah
0040:0071 The top bit of the byte is set when BIOS detects a CONTROL-BREAK . The
PC's system software never clears this bit
0040:0078 Table of four bytes contains the time-out value for each LPT port
0040:007C Table of four bytes contains time-out value for each Com port
0040:0080 Word points to the beginning of keyboard type-ahead buffer which must
be somewhere within segment 0040H.By default this location is 001EH. Some BIOS
cloners don't use this address themselves; they provide it for programs to
examine. However, many BIOS chips(including IBMBIOS) allow programs to change
this address
0040:0082 This word points the end of the keyboard buffer
0040:00F0 PC and AT reserve the 16 bytes starting at this address for
communications between different programs
0050:0000 This byte contains the status of the PRINT SCREEN command (INT 05h).
If it is 0 the last print screen was successful. If it was a FFH an error
occurred. If the byte is 1 print screen in progress. This a handy way to disable
the key. The BIOS will think print screen is already in progress.
Understanding Computers OR Computer Operating Systems
need other links go to SITE MAP
home B & I SNOWDEN-Find a Wealth of Products and Services