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| Counting RAM using the 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 Browns Interrupt List:)
--------B-1588-------------------------------
INT 15 - SYSTEM - GET EXTENDED MEMORY SIZE (286+)
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: DOS TSRs which wish to allocate extended memory for 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. RFC: that TSR-thing is irrelevant for OS-programming, imho
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
SeeAlso:AH=87h,AH=8Ah"Phoenix",AH=C7h,AX=DA88h,AX=E801h,AX=E820h
--------b-15E820-----------------------------
INT 15 - newer BIOSes - GET SYSTEM MEMORY MAP
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 BIOS as reserved standard PC address ranges will not be reported
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
sessions)
other not defined yet -- treat as Reserved
SeeAlso: #00559 |
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