What is the A20 line?

The A20 line takes a bit of explaining so hang with me please.

When the AT was introduced, it was able to access up to sixteen megabytes of memory, but in order to remain compatible with the IBM-XT, due to a quirk in the XT architecture (memory wraparound) had to be duplicated in the AT so things would work the same, the 20th address line on the bus (A20) was turned off so this "wrap-around" effect worked and software from the old XT days continued to work (compatibility was a *B*I*G* issue back then).

The A20 line is controlled by the keyboard controller unit. Which is usually a derivative of the 8042 chip.


Why can't I access all my memory! I have 128mb and can only use 1mb?

The main reason you can't access all your memory (or only "odd" megabytes) is because you need to enable the A20 line on your bus. (For an explanation of the A20 line, see What is the A20 line?


How do I enable the A20 line?

To enable the A20 line, you have to use some hardware IO using the Keyboard Controller chip (8042 chip) and enable it. Good documentation exists for the 8042 chip but here is my source for enabling the A20 in C code.

The flow chart for this is;

  1. Disable interrupts
  2. Wait until the keyboard buffer is empty
  3. Send command to disable the keyboard
  4. Wait until the keyboard buffer is empty
  5. Send command to read output port
  6. Wait until the keyboard buffer is empty
  7. Save byte from input port
  8. Wait until the keyboard buffer is empty
  9. Send command Write output port
  10. Wait until the keyboard buffer is empty
  11. Send saved byte OR by 2 (GATE A20 to ON)
  12. Wait until the keyboard buffer is empty
  13. Enable the keyboard
  14. Enable interrupts

Here is my C source;

/* Init the A20 by Dark Fiber */
void init_A20(void)


	kyb_send_command(0xAD);		// disable keyboard

	kyb_send_command(0xD0);		// Read from input


	kyb_send_command(0xD1);		// Write to output


	kyb_send_command(0xAE);		// enable keyboard


Or in ASM if you wish

;; NASM 32bit assembler

[bits 32]
[section .text]


	call    a20wait
	mov     al,0xAD
	out     0x64,al

	call    a20wait
	mov     al,0xD0
	out     0x64,al

	call    a20wait2
	in      al,0x60
	push    eax

	call    a20wait
	mov     al,0xD1
	out     0x64,al

	call    a20wait
	pop     eax
	or      al,2
	out     0x60,al

        call    a20wait
	mov     al,0xAE
	out     0x64,al

	call    a20wait

.l0:	mov     ecx,65536
.l1:	in      al,0x64
	test    al,2
	jz      .l2
	loop    .l1
	jmp     .l0
.l2:	ret

.l0:	mov     ecx,65536
.l1:	in      al,0x64
	test    al,1
	jnz     .l2
        loop    .l1
	jmp     .l0
.l2:	ret


How do I determine RAM size?

Determining how much memory you have is one of the first things that most people implement in their kernel. In the "old" days of operating systems this was very easy as few people had more than 64mb of memory.

Why did I mention 64mb of memory? Because that's all the CMOS can hold up to 99mb values. When you encounter machines with 128mb what are you going to do? There are some functions in the BIOS for memory handling. The first set of calls supported by all BIOS only returns what is in the CMOS, thus making it irrelevant. There is some more advanced calls but are not guaranteed to be in every BIOS you encounter, and since not every machine supports it, means its also not what we want. Two other methods people use are directly probing memory, and using your motherboard chipset registers to determine memory. The only drawback with the later methos is you must know what kind of chipset the user has on his motherboard....

I prefer to directly probe memory at every 1mb interval and test for memory that way.

Something to note, not that you may need to know, is that on old machines (maybe even new ones), it is possible to have less than 640kb base memory and still have extended memory beyond the 1mb mark. But in today's machines where most people have single 64mb DIMM's, you wont come across this.


How do I determine RAM size with BIOS?

You can determine RAM size with the BIOS via two different calls.

The first call is built in nearly every BIOS, the later call is only contained within newer BIOS's

(from Ralf Brown's Interrupt Listing)
  AH = 88h
  Return: CF clear if successful
    AX = number of contiguous KB starting at absolute address 100000h
    CF set on error
  AH = status
    80h invalid command (PC,PCjr)
    86h unsupported function (XT,PS30)

Notes: TSRs which wish to allocate extended memory to themselves often hook this call, and return a reduced memory size. They are then free to use the memory between the new and old sizes at will.

the standard BIOS only returns memory between 1MB and 16MB; use AH=C7h for memory beyond 16MB not all BIOSes correctly return the carry flag, making this call unreliable unless one first checks whether it is supported through a mechanism other than calling the function and testing CF


  AX = E820h
  EAX = 0000E820h
  EDX = 534D4150h ('SMAP')
  EBX = continuation value or 00000000h to start at beginning of map
  ECX = size of buffer for result, in bytes (should be >= 20 bytes)
  ES:DI -> buffer for result (see #00560)
  Return: CF clear if successful
    EAX = 534D4150h ('SMAP')
    ES:DI buffer filled
    EBX = next offset from which to copy or 00000000h if all done
    ECX = actual length returned in bytes
    CF set on error
      AH = error code (86h) (see #00475 at INT 15/AH=80h)

Notes: originally introduced with the Phoenix BIOS v4.0, this function is now supported by most newer BIOSes, since various versions of Windows call it to find out about the system memory a maximum of 20 bytes will be transferred at one time, even if ECX is higher; some BIOSes (e.g. Award Modular BIOS v4.50PG) ignore the value of ECX on entry, and always copy 20 bytes some BIOSes expect the high word of EAX to be clear on entry, I.e. EAX=0000E820h

if this function is not supported, an application should fall back to AX=E802h, AX=E801h, and then AH= 88h the BIOS is permitted to return a nonzero continuation value in EBX and indicate that the end of the list has already been reached by returning with CF set on the next iteration this function will return base memory and ISA/PCI memory contiguous with base memory as normal memory ranges; it will indicate chipset-defined address holes which are not in use and motherboard memory-mapped devices, and all occurrences of the system BIOSasreserved;standardPCaddressrangeswillnotbereported

SeeAlso:AH=C7h,AX=E801h"Phoenix",AX=E881h,MEM xxxxh:xxx0h"ACPI"

Format of Phoenix BIOS system memory map address range descriptor:
Offset	Size	Description	(Table 00559)
 00h	QWORD	base address
 08h	QWORD	length in bytes
 10h	DWORD	type of address range (see #00560)

(Table 00560)
Values for System Memory Map address type:
  01h	memory, available to OS
  02h	reserved, not available (e.g. system ROM, memory-mapped device)
  03h	ACPI Reclaim Memory (useable by OS after reading ACPI tables)
  04h	ACPI NVS Memory (OS is required to save this memory between NVS
 other	not defined yet -- treat as Reserved
 SeeAlso: #00559


How do I determine RAM size with direct probing?

Directly probing memory, being a method that does not rely on the BIOS makes it more portable than one that does rely on the BIOS. Depending on how its coded, may or may not take into account holes in system memory (15/16mb mark ala OS/2) or memory mapped devices like frame buffering SVGA cards, etc.
 * void count_memory (void)
 * probes memory above 1mb
 * last mod : 05sep98 - stuart george
 *            08dec98 - ""     ""
 *            21feb99 - removed dummy calls
void count_memory(void)
	register ULONG *mem;
	ULONG	mem_count, a;
	USHORT	memkb;
	UCHAR	irq1, irq2;
	ULONG	cr0;

	/* save IRQ's */

	/* kill all irq's */
	outb(0x21, 0xFF);
	outb(0xA1, 0xFF);


	// store a copy of CR0
	__asm__ __volatile("movl %%cr0, %%eax":"=a"(cr0))::"eax");

	// invalidate the cache
	// write-back and invalidate the cache
	__asm__ __volatile__ ("wbinvd");

	// plug cr0 with just PE/CD/NW
	// cache disable(486+), no-writeback(486+), 32bit mode(386+)
	__asm__ __volatile__("movl %%eax, %%cr0", :: "a" (cr0 | 0x00000001 | 0x40000000 | 0x20000000) : "eax");



		// the empty asm calls tell gcc not to rely on whats in its registers
		// as saved variables (this gets us around GCC optimisations)

	}while(memkb<4096 && mem_count!=0);

	__asm__ __volatile__("movl %%eax, %%cr0", :: "a" (cr0) : "eax");


	outb(0x21, irq1);
	outb(0xA1, irq2);