Memory map

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A hypothetical memory map of bank-switched memory for a processor that can only address 64 kB. This scheme shows 200 kB of memory, of which only 64 kb can be accessed at any time by the processor. The operating system must manage the bank switching operation to ensure that program execution can continue when part of memory is not accessible to the processor.

In computer science, a memory map is a structure of data (which usually resides in memory itself) that indicates how memory is laid out. Memory maps can have a different meaning in different parts of the operating system.

In the boot process, a memory map is passed on from the firmware in order to instruct an operating system kernel about memory layout. It contains the information regarding the size of total memory, any reserved regions and may also provide other details specific to the architecture.

In virtual memory implementations and memory management units, a memory map refers to page tables, which store the mapping between a certain process's virtual memory layout and how that space relates to physical memory addresses.

In native debugger programs, a memory map refers to the mapping between loaded executable/library files and memory regions. These memory maps are used to resolve memory addresses (such as function pointers) to actual symbols.


BIOS Memory map[edit]

The PC BIOS provides a set of routines that can be used by operating system to get memory layout. Some of the available routines are:

BIOS Function: INT 0x15, AX=0xE801 [1]:

This BIOS interrupt call is used by the running OS to get the memory size for 64MB+ configurations. It is supported by AMI BIOSses dated 8/23/94 or later. The operating system just sets AX to 0xE801 then calls int 0x15. If some error has happened, the routine returns with CF (Carry Flag) set to 1. If no error, the routine returns with CF clear and the state of registers is described as following:

Register Value Description
EAX Extended memory between 1M and 16M, in KB. (Maximum value: 0x3C00 = 15360KB).
EBX Count of 64K blocks above 16M.
ECX Configured memory 1M to 16M, in KB.
EDX Count of configured 64K blocks above 16M.

BIOS Function: INT 0x15, AX=0xE820 - GET SYSTEM MEMORY MAP [2]:

Input:

Register Value Description
EAX 0x0000E820
EBX Continuation value or 0x0 to start at beginning of map.
ECX Size of buffer for result (20 bytes).
EDX 0x534D4150 (ASCII for 'SMAP')
ES:DI segment:offset location of the buffer for result.

SMAP buffer structure:

Offset Size Value Description
0 QWORD (8 bytes) Base address of memory map region referred to by EBX.
8 QWORD (8 bytes) Length in bytes.
16 DWORD (4 bytes) Type of address range.

How used: The operating system shall allocate an SMAP buffer in memory (20 bytes buffer). Then set registers as specified in "Input" table. On first call, EBX should be set to 0. Next step is to call INT 0x15. If no error, the interrupt call returns with CF clear and the buffer filled with data representing first region of the memory map. EBX is updated by BIOS so that when the OS calls the routine again, The next region is returned in the buffer. BIOS sets EBX is to zero if all done.

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