The 3 methods for enabling the A20 Gate

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When IBM PC AT System was introduced ,the new Intel 286 processor was not compatible with the old x86 processor.The older x86 micro-processors(Intel 8086) had address bus of 20bits which would total and give access up to 1megabyte of memory.The Intel 386 and above had address bus up to 32 bits allowing 4 Gigabytes of memory.But the old 8086 processors did not have such a big address bus.To keep in compatible with the older processors and solve the problem the Intel introduced a logical OR gate at the 20 bit of the address bus which could be enabled a or disabled.So, to keep in compatible with the older processors and programs the A20 is disabled at the startup. NOTE:BIOS actually enables the A20 for counting and testing the available memory and then disables it before booting again to keep in compatible with older processors. The A20 gate is an electronic OR gate which can be disabled and enabled and placed at the 20th bit of the address bus.It is connected through a P21 line of the keyboard controller which made it possible for the keyboard controller to enable or disable the A20 Gate. In the current generations, there is a need to have memory lot more than just 1MB.The applications, games,etc need a lot of memory.Even a operating system kernel might eat up the whole 1MB.So it is something like impossible to run modern programs in 1MB of memory.Looks like the A20 is an important feature for good functional of the operating system. To enable the A20 gate there are 3 methods or you can skip this step by using the high memory managers such as HIMEM.sys or using bootloaders such as GRUB(GRUB will set up you up with protected mode with A20 enabled) The 3 methods for enabling the A20 Gate are 1. Keyboard Controller 2. BIOS Function 3. System Port =============================================================================================== Keyboard Controller: This is the most common method of enabling A20 Gate.The keyboard micro-controller provides functions for disabling and enabling A20.Before enabling A20 we need to disable interrupts to prevent our kernel from getting messed up.The port 0x64 is used to send the command byte. Command Bytes and ports 0xDD Enable A20 Address Line 0xDF Disable A20 Address Line 0x64 Port of the 8042 micro-controller for sending commands Using the keyboard to enable A20: EnableA20_KB: cli ;Disables interrupts push ax ;Saves AX mov al, 0xdd ;Look at the command list out 0x64, al ;Command Register pop ax ;Restore's AX sti ;Enables interrupts ret Using the BIOS functions to enable the A20 Gate: The INT 15 2400,2401,2402 are used to disable,enable,return status of the A20 Gate respectively. Return status of the commands 2400 and 2401(Disabling,Enabling) CF = clear if success AH = 0 CF = set on error AH = status (01=keyboard controller is in secure mode, 0x86=function not supported) Return Status of the command 2402 CF = clear if success AH = status (01: keyboard controller is in secure mode; 0x86: function not supported) AL = current state (00: disabled, 01: enabled) CX = set to 0xffff is keyboard controller is no ready in 0xc000 read attempts CF = set on error Disabling the A20 push ax mov ax, 0x2400 int 0x15 pop ax Enabling the A20 push ax mov ax, 0x2401 int 0x15 pop ax Checking A20 push ax push cx mov ax, 0x2402 int 0x15 pop cx pop ax ===================================================================================== Using System Port 0x92 This method is quite dangerous because it may cause conflicts with some hardware devices forcing the system to halt. Port 0x92 Bits

Bit 0 - Setting to 1 causes a fast reset
Bit 1 - 0: disable A20, 1: enable A20
Bit 2 - Manufacturer defined
Bit 3 - power on password bytes. 0: accessible, 1: inaccessible
Bits 4-5 - Manufacturer defined
Bits 6-7 - 00: HDD activity LED off, 01 or any value is "on"

Code to enable A20 through port 0x92 push ax mov al, 2 out 0x92, al pop ax