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8086 CPU can access up to 1 MB of random access memory (RAM), it is limited by segment/offset construction. Since segment registers (CS, SS, ES, DS) can hold maximum value of 0FFFFh and offset registers (IP, BX, SI, DI, BP, SP) can also hold maximum value of 0FFFFh, the largest logical memory location that we can access is FFFF:FFFF or physical address: 0FFFFh * 10h + 0FFFFh = 10FFEFh = 65535 * 16 + 65535 = 1,114,095 bytes
Modern processors have a larger registers so they have much larger memory area that can be accessed, but the idea is still the same.



Memory Table of Emulator (and typical IBM PC):

Physical address of memory area in HEX Short Description
00000 - 00400 Interrupt vectors. Emulator loads "INT_VECT" file at the physical address 00000h.
00400 - 00500 System information area. We use a trick to set some parameters by loading a tiny last part (21 bytes) of "INT_VECT" in that area (the size of that file is 1,045 or 415h bytes, so when loaded it takes memory from 00000 to 00415h).
This memory block is updated by emulator when configuration changes, see System information area table.
00500 - A0000 A free memory area. A block of 654,080 bytes. Here you can load your programs.
A0000 - B1000 Video memory for VGA, Monochrome, and other adapters. Not used by emulator!
B1000 - B8000 Reserved. Not used by emulator!
B8000 - C0000 32 KB video memory for Color Graphics Adapter (CGA). Emulator uses this memory area to keep 8 pages of video memory. The Emulator screen can be resized, so less memory is required for each page, although emulator always uses 1000h (4096 bytes) for each page (see INT 10h / AH=05h in the list of supported interrupts).
C0000 - F4000 Reserved.
F4000 - 10FFEF ROM BIOS and extensions. Emulator loads "BIOS_ROM" file at the physical address 0F4000h. Interrupt table points to this memory area to get emulation of interrupt functions.


Interrupt Vector (memory from 00000h to 00400h)

INT number       Address in              Address of
in hex           Interrupt Vector        BIOS sub-program

00               00x4 = 00               F400:0170 - CPU-generated,
                                                     divide error.

04               04x4 = 10               F400:0180 - CPU-generated,
                                                     INTO detected
                                                     overflow.

10               10x4 = 40               F400:0190 - Video functions.

11               11x4 = 44               F400:01D0 - Get BIOS
                                                     equipment list.

12               12x4 = 48               F400:01A0 - Get memory size.

13               13x4 = 4C               F400:01B0 - Disk functions.

15               15x4 = 54               F400:01E0 - BIOS functions.

16               16x4 = 58               F400:01C0 - Keyboard functions.

19               19x4 = 64               FFFF:0000 - Reboot.

1A               1Ax4 = 68               F400:0160 - Time functions.

1E               1Ex4 = 78               F400:AFC7 - Vector of Diskette
                                                     Controller Params.

20               20x4 = 80               F400:0150 - DOS function:
                                                     terminate program.

21               21x4 = 84               F400:0200 - DOS functions.

all others       ??x4 = ??               F400:0100 - The default interupt
                                                     catcher. Prints out
                                                     "Interupt not supported
                                                     yet" message.
A call to BIOS sub-system is disassembled by "BIOS DI" (it doesn't use DI register in any way, it's just because of the way the encoding is done: we are using "FF /7" for such encoding, "FFFFCD10" is used to make emulator to emulate interrupt number 10h).

F400:0100 has this code FFFFCDFF (decoded as INT 255, and error message is generated).



 
System information area (memory from 00400h to 00500h)

Address (hex) Size Description
0040h:0010 WORD BIOS equipment list.

Bit fields for BIOS-detected installed hardware:
Bit(s)	Description
 15-14  number of parallel devices.
 13     not supported.
 12     game port installed.
 11-9   number of serial devices.
 8      reserved.
 7-6    number of floppy disk drives (minus 1):
          00 single floppy disk;
          01 two floppy disks;
          10 three floppy disks;
          11 four floppy disks.
 5-4    initial video mode:
          00 EGA,VGA,PGA, or other with on-board video BIOS;
          01 40x25 CGA color;
          10 80x25 CGA color (emulator default);
          11 80x25 mono text.
 3    not supported.
 2    not supported.
 1    math coprocessor installed.
 0    set when booted from floppy (always set by emulator).
This word is also returned in AX by INT 11h.
Default value: 0021h or 0000 0000 0010 0001b
0040h:0013 WORD Kilobytes of contiguous memory starting at absolute address 00000h.
This word is also returned in AX by INT 12h.
This value is set to: 0280h (640KB).
0040h:004A WORD Number of columns on screen.
Default value: 0032h (50 columns).
0040h:004E WORD Current video page start address in video memory (after 0B800:0000).
Default value: 0000h.
0040h:0050 8 WORDs Contains row and column position for the cursors on each of eight video pages.
Default value: 0000h (for all 8 WORDs).
0040h:0062 BYTE Current video page number.
Default value: 00h (first page).
0040h:0084 BYTE Rows on screen minus one.
Default value: 13h (19+1=20 columns).



See also: Custom Memory Map
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