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Global Memory Table
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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