Understanding POST and ROM-BIOS service functions

1
Understanding POST and ROM-BIOS service functions

• Numerous low-level services are available to
  real-mode programs (include boot-loaders)

2
Power-On Self-Test (POST)

• When computer is first turned on, the CPU starts
  executing instructions in ROM-BIOS
• Reset-address CS0xF000, EIP0xFFF0
• Initial routines perform vital functions
• Verify amount of installed read/write memory
• Setup the table of real-mode interrupt-vectors
• Detect and initialize the peripheral equipment
• Setup parameters in the BIOS DATA AREA
• Read in, and transfer to, the boot-loader

3
Memory Layout during POST
The POST creates the Interrupt Vector Table and
fills in parameters of the Bios Data area
Beginning
Finished
RAM
ROM-BIOS
RAM
ROM-BIOS
VRAM
VRAM
1MB
POST finishes up by reading in the boot-loader
from a storage device
Boot-loader
BIOS DATA
IVT
4
Two demo-programs

• The program showivt.cpp lets you look at the
  table of (real-mode) interrupt vectors (addresses
  0x00000000 to 0x00000400)
• The program showrbda.cpp lets you see the
  parameter-values that the ROM-BIOS has stored in
  its ROM-BIOS DATA-AREA (addresses 0x00000400 to
  0x00000500)
• A special device-driver (named dos.o) is needed
  for accessing these memory areas

5
ROM-BIOS service-functions

• During POST, the startup routines need to use
  various low-level system-services
• To show diagnostic messages on the display
• To accept keystroke-commands from a user
• To query the real-time clock/calendar device
• To optionally send important data to a printer
• To read in a data-sector from the boot device
• The actions are performed as subroutines

6
Space in ROM is tight

• Size of a typical ROM-BIOS chip is 64KB
• Needs to store the instructions and data for
  several hundred different functions
• So cannot afford to waste precious space
• Routines are written in assembly language
• Clever coding is used to optimize storage usage
  (e.g., ROM functions get invoked by
  software-interrupt instructions (2-bytes) as an
  alternative to subroutine-calls (3-bytes)

7
Services remain available

• All of the low-level ROM-BIOS services remain
  available to Real-Mode programs
• This includes boot-loader programs
• If were going to write our own boot-loader, we
  will benefit by knowing what low-level services
  are already available in ROM (as we will face
  tight space limitations, too namely, the
  512-byte disk sector-size)

8
Catalog of ROM-BIOS services

• int 0x10 video display services
• int 0x11 equipment-list service
• int 0x12 memory-size service
• int 0x13 disk input/output services
• int 0x14 serial communications services
• int 0x15 system software services

9
ROM-BIOS services (continued)

• int 0x16 keyboard input/control services
• int 0x17 parallel-port printer services
• int 0x18 diskless bootstrap service
• int 0x19 system reboot service
• int 0x1A real-time clock services

10
Example Get Memory Size

• When its time to load an operating system the
  OS-loader will need to know where it can place
  the OS code and data so as to efficiently fit
  within the systems memory
• So the question will be How much actual ram is
  available?
• One of the simplest ROM-BIOS services can be
  invoked to get the answer.

11
Calling Get Memory Size

• No service-parameters are required
• Just execute an int 0x12 instruction
• Size of the available ram is returned in AX
• (expressed in kilobytes 1KB1024 bytes)
• Program-code looks like this int 0x12
  call BIOS service mov kb_ram, ax save
  return-value

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How does it work?

• Executing int 0x12 transfers control to an
  Interrupt Service Routine within the ROM
• The CPU gets the entry-point for this ISR from
  the Interrupt Vector Table (IVT)
• The IVT has enough room for 256 vectors (all
  vectors use a doubleword of storage)
• The number 0x12 is an array-index for IVT
• Example vector 0x12 equals 0xF000F841

13
Heres the ISRs code

• isr_0x12
• push ds preseve DS
• mov ax, 0x40 address BD area
• mov ds, ax using DS register
• mov ax, 0x0013 get POST param
• pop ds restore DS
• iret return from ISR

14
Demo Program memsize.s

• We wrote a simple boot-sector program to
  illustrate the Get Memory Size service
• It executes int 0x12 to obtain the amount of
  usable real-mode memory (in kilobytes)
• It converts the binary value obtained in AX to
  its decimal representation as a string of ascii
  characters, and int 0x10 functions to display
  the information in readable form

15
In-Class Exercise 1

• Write a similar boot-sector demo program that
  will display the Equipment-Check List (a bitmap
  showing some installed devices thats returned in
  AX if int 0x11 executes)
• You can look at our showmsw.s demo as a guide
  to writing your assembler code

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Equipment List Bitmap

1. 14 13 12 11 10 9 8 7 6
   5 4 3 2 1 0

Internal modem (1yes, 0no)
Number of printer-ports
Number of serial-ports
Number of diskette drives (if bit 0 is set)
(001 drive, 012 drives,
etc)
Initial video-display mode (1180x25 monochrome,
1080x25 color, 0140x25 color, 00EGA/VGA/SVGA)
PS/2-type pointing-device is installed (1yes,
0no)
External math-coprocessor installed (1yes, 0no)
Diskette available for booting (1yes, 0no)
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In-Class Exercice 2

• Enhance the showmem.s demo-program by showing
  additional information about the amount of
  physical memory installed use ROM-BIOS
  system-services int 0x15 function 0xE8,
  subfunction 0x01 (see Ralf Browns online
  Interrupt-List for details)

18
Extended Memory Areas
mov ax, 0xE801 int 0x15
Pentiums Memory-area (4GB)
Number of 64KB blocks Is reported by ROM-BIOS
(in register BX)
80286 memory-area (16MB)
Number of 1KB blocks Is reported by ROM-BIOS
(in register AX)
8086 memory-area (1MB)