ROM BIOS and the first MEG of memory
ROM BIOS and the first MEG of memory
The following is the layout and contents of the first Meg of memory. As I get more information, I will update it accordingly.
| Notes | Offset (hex) | Size (dec) | Description |
| 0000 | 4 | Interrupt Vector for INT 00h (division by zero) | |
| 0004 | 4 | Interrupt Vector for INT 01h (single step) | |
| 0008 | 4 | Interrupt Vector for INT 02h (Nonmaskable) | |
| 000C | 4 | Interrupt Vector for INT 03h (Break point) | |
| 0010 | 4 | Interrupt Vector for INT 04h (overflow) | |
| 0014 | 4 | Interrupt Vector for INT 05h (print screen) | |
| 0018 | 4 | Interrupt Vector for INT 06h (invalid opcode) | |
| 001C | 4 | Interrupt Vector for INT 07h (processor extension not available) | |
| 0020 | 4 | Interrupt Vector for INT 08h (clock tick (hardware)) | |
| 0024 | 4 | Interrupt Vector for INT 09h (keyboard) | |
| 0028 | 4 | Interrupt Vector for INT 0Ah (LPT2) | |
| 002C | 4 | Interrupt Vector for INT 0Bh (COM2) | |
| 0030 | 4 | Interrupt Vector for INT 0Ch (COM1) | |
| 0034 | 4 | Interrupt Vector for INT 0Dh (IRQ5) | |
| 0038 | 4 | Interrupt Vector for INT 0Eh (floppy disk attention) | |
| 003C | 4 | Interrupt Vector for INT 0Fh (printer control) | |
| 0040 | 4 | Interrupt Vector for INT 10h (video) | |
| 0044 | 4 | Interrupt Vector for INT 11h (equipment list) | |
| 0048 | 4 | Interrupt Vector for INT 12h (memory size) | |
| 004C | 4 | Interrupt Vector for INT 13h (disk BIOS) | |
| 0050 | 4 | Interrupt Vector for INT 14h (communications BIOS) | |
| 0054 | 4 | Interrupt Vector for INT 15h (system BIOS) | |
| 0058 | 4 | Interrupt Vector for INT 16h (keyboard BIOS) | |
| 005C | 4 | Interrupt Vector for INT 17h (printer BIOS) | |
| 0060 | 4 | Interrupt Vector for INT 18h (ROM Basic Language) | |
| 0064 | 4 | Interrupt Vector for INT 19h (boot-strap) | |
| 0068 | 4 | Interrupt Vector for INT 1Ah (Time and Date) | |
| 006C | 4 | Interrupt Vector for INT 1Bh (Ctrl Break) | |
| 0070 | 4 | Interrupt Vector for INT 1Ch (Clock tick) | |
| 0074 | 4 | Holds address pointing to control parameters | |
| 0078 | 4 | Holds address pointing to floppy disk drive parameter table | |
| 007C | 4 | Holds address pointing to second half of character table (128 - 255) | |
| 0080 | 4 | Interrupt Vector for INT 20h (terminate) | |
| 0084 | 4 | Interrupt Vector for INT 21h (MS-DOS function calls) | |
| 0088 | 4 | Holds address pointing to MS-DOS program terminate routine | |
| 008C | 4 | Holds address pointing to MS-DOS keyboard-break handler | |
| 0090 | 4 | Holds address pointing to MS-DOS critical-error handler | |
| 0094 | 4 | Interrupt Vector for INT 25h (Absolute disk read) | |
| 0098 | 4 | Interrupt Vector for INT 26h (Absolute disk write) | |
| 009C | 4 | Interrupt Vector for INT 27h (MS-DOS TSR) | |
| 00A0 | 4 | Interrupt Vector for INT 28h (MS-DOS IDLE) | |
| 00A4 | 4 | Interrupt Vector for INT 29h (Fast CON output) | |
| 00A8 | 4 | Interrupt Vector for INT 2Ah (NetBIOS) | |
| 00A8 | 12 | Interrupt Vector for INT 2Bh-2Dh (reserved) | |
| 00B8 | 4 | Interrupt Vector for INT 2Eh (Execute) | |
| 00BC | 4 | Interrupt Vector for INT 2Fh (Multitask) | |
| 00C0 | 4 | (NOT A VECTOR!) - DOS 1+ - FAR JMP instruction | |
| 00C4 | 4 | overwritten by CP/M jump instruction in INT 30 | |
| 00C8 | 4 | Interrupt Vector for INT 32h (user) | |
| 00CC | 4 | Interrupt Vector for INT 33h (Mouse) | |
| 00D0 | 44 | Interrupt Vector for INT 34h-3Eh (FLOATING POINT EMULATION) | |
| 00FC | 4 | Interrupt Vector for INT 3Fh (Overlay manager) | |
| 0100 | 4 | Interrupt Vector for INT 40h (Disk handler) | |
| 0104 | 4 | Holds address pointing to Parameter table for first hard drive | |
| 0108 | 4 | Interrupt Vector for INT 42h (video) | |
| 010C | 4 | Holds address pointing to video graphics characters (EGA, PS/2s) | |
| 0110 | 4 | Holds address pointing to video graphics characters (PCjr) | |
| 0114 | 4 | Z100/Acron | |
| 0118 | 4 | Holds address pointing to Parameter table for second hard drive | |
| 011C | 4 | Z100/Acorn/Western Digital/SQL Base | |
| 0120 | 4 | KEYBOARD (PCjr) | |
| 0124 | 4 | SYSTEM DATA (PCjr) | |
| 0128 | 4 | Interrupt Vector for INT 4Ah (USER ALARM HANDLER) | |
| 012C | 4 | Interrupt Vector for INT 4Bh (IBM SCSI interface) | |
| 0130 | 4 | Interrupt Vector for INT 4Ch (Z100/Acorn/TI) | |
| 0134 | 4 | Interrupt Vector for INT 4Dh (Z100) | |
| 0138 | 4 | Interrupt Vector for INT 4Eh (Z100/TI) | |
| 013C | 4 | Interrupt Vector for INT 4Fh (Common Access Method SCSI) | |
| 0140 | 4 | Interrupt Vector for INT 50h (IRQ0 relocated by software) | |
| 0144 | 4 | Interrupt Vector for INT 51h (IRQ1 relocated by software) | |
| 0148 | 4 | Interrupt Vector for INT 52h (IRQ2 relocated by software) | |
| 014C | 4 | Interrupt Vector for INT 53h (IRQ3 relocated by software) | |
| 0150 | 4 | Interrupt Vector for INT 54h (IRQ4 relocated by software) | |
| 0154 | 4 | Interrupt Vector for INT 55h (IRQ5 relocated by software) | |
| 0158 | 4 | Interrupt Vector for INT 56h (IRQ6 relocated by software) | |
| 015C | 4 | Interrupt Vector for INT 57h (IRQ7 relocated by software) | |
| 0160 | 4 | Interrupt Vector for INT 58h (IRQ8/0 relocated by software) | |
| 0164 | 4 | Interrupt Vector for INT 59h (IRQ9/1 relocated by software) | |
| 0168 | 4 | Interrupt Vector for INT 5Ah (IRQ10/2 relocated by software) | |
| 016C | 4 | Interrupt Vector for INT 5Bh (IRQ11/3 relocated by software) | |
| 0170 | 4 | Interrupt Vector for INT 5Ch (IRQ12/4 relocated by software) | |
| 0174 | 4 | Interrupt Vector for INT 5Dh (IRQ13/5 relocated by software) | |
| 0178 | 4 | Interrupt Vector for INT 5Eh (IRQ14/6 relocated by software) | |
| 017C | 4 | Interrupt Vector for INT 5Fh (IRQ15/7 relocated by software) | |
| 0180 | 28 | Interrupt Vector for INT 60h-66h (reserved for user interrupt) | |
| 019C | 4 | Interrupt Vector for INT 67h (LIM Expanded Memory Manager) | |
| 01A0 | 16 | Interrupt Vector for INT 68h-6Bh (reserved for user interrupt) | |
| 01B0 | 4 | Interrupt Vector for INT 6Ch (DECnet DOS network scheduler) | |
| 01B4 | 4 | Interrupt Vector for INT 6Dh (VGA internal) | |
| 01B8 | 4 | Interrupt Vector for INT 6Eh (DECnet DOS) | |
| 01BC | 4 | Interrupt Vector for INT 6Fh (Novell NetWare, MS Windows 3.0) | |
| 01C0 | 4 | Interrupt Vector for INT 70h (CMOS real-time clock) | |
| 01C4 | 4 | Interrupt Vector for INT 71h (redirected to int 0A) | |
| 01C8 | 8 | Interrupt Vector for INT 72h-73h (IRQ10-11 reserved) | |
| 01D0 | 4 | Interrupt Vector for INT 74h (IRQ12 Pointing device) | |
| 01D4 | 4 | Interrupt Vector for INT 75h (IRQ13 math coprocessor exception) | |
| 01D8 | 4 | Interrupt Vector for INT 76h (IRQ14 Hard disk controller) | |
| 01DC | 4 | Interrupt Vector for INT 77h (IRQ15 reserved) | |
| 01E0 | 4 | Interrupt Vector for INT 78h (DOS extenders?) | |
| 01E4 | 4 | Interrupt Vector for INT 79h (reserved for user interrupt) | |
| 01E8 | 4 | Interrupt Vector for INT 7Ah (Novell NetWare) | |
| 01E8 | 20 | Interrupt Vector for INT 7Bh-7Fh (reserved for user interrupt) | |
| 0200 | 24 | Interrupt Vector for INT 80h-85h (reserved for BASIC) | |
| 0218 | 420 | Interrupt Vector for INT 86h-EEh (reserved for BASIC) | |
| 03BC | 4 | Interrupt Vector for INT EFh (BASIC - orig. INT 09 vector) | |
| 03C0 | 4 | Interrupt Vector for INT F0h (BASIC - orig. INT 08 vector) | |
| 03C4 | 52 | Interrupt Vector for INT F1h-FDh (reserved for user interrupt) | |
| 03F8 | 8 | Interrupt Vector for INT FEh-FFh (AT/XT286/PS50+) | |
| Remember that 0000:0400h = 0040:0000h | |||
| 0400 | 2 | COM1 I/O base address (set to 0 if none found) | |
| 0402 | 2 | COM2 I/O base address (set to 0 if none found) | |
| 0404 | 2 | COM3 I/O base address (set to 0 if none found) | |
| 0406 | 2 | COM4 I/O base address (set to 0 if none found) | |
| 0408 | 2 | LPT1 I/O base address (set to 0 if none found) | |
| 040A | 2 | LPT2 I/O base address (set to 0 if none found) | |
| 040C | 2 | LPT3 I/O base address (set to 0 if none found) | |
| 040E | 2 | LPT4 I/O base address (set to 0 if none found) | |
| 2 | (if extended BIOS on system) Segment address of Extended BIOS Data Area (9FC0:0000h) | ||
| 0410 | 2 | equipment list reported by interrupt 11h (see figure 1) | |
| 0412 | 1 | PCjr - Infrared Keyboard Link Error | |
| 0413 | 2 | usable memory size in kilobytes (interrupt 12h) | |
| Usually set to 0280h. If less, then denotes 640k-difference=size of Extended BIOS Data Area at (9FC0:0000h) | |||
| 0415 | 2 | (AT+) Error codes from the BIOS manufacturer | |
| 2 | (PC,XT) Adapter Memory Size | ||
| 0417 | 2 | keyboard status bits (see figure 2) | |
| 0419 | 1 | Alternate Keypad entry | |
| 041A | 2 | points to current head of keyboard buffer at 0040:001Eh | |
| 041C | 2 | points to current tail of keyboard buffer at 0040:001Eh | |
| 041E | 32 | keyboard buffer - used as sixteen 2-byte entries | |
| 043E | 1 | indicates whether a floppy drive should be re calibrated | |
| 043F | 1 | floppy drive motor status (bites 0-3) | |
| bit 0 = drive 0, if set - drive motor running | |||
| bit 1 = drive 1, if set - drive motor running, etc. | |||
| 0440 | 1 | floppy drive motor status. The value in this byte is | |
| decremented with every tick of the system clock. | |||
| When the value returns 0, the BOIS turns off the motor | |||
| 0441 | 1 | floppy drive motor status. status code returned by the | |
| ROM BIOS from the most recent disk operation | |||
| 0442 | 7 | floppy drive controller status | |
| 0449 | 1 | current video mode | |
| 044A | 2 | max number of chars in a row of text | |
| 044C | 2 | number of bytes needed to display the screen (text=80x25) | |
| 044E | 2 | offset of current display page | |
| 0450 | 16 | 8 words, each giving the cursor location of each page | |
| 0460 | 2 | size of current cursor | |
| 0462 | 1 | current display page | |
| 0463 | 2 | port address of the hardware display controller chip | |
| 0465 | 1 | current setting of the display mode-register of the MDA and CGA | |
| 0466 | 1 | current setting of the CGA's CRT color register | |
| 0467 | 5 | cassette tape control | |
| 4 | Reset Restart Address | ||
| 046C | 4 | master clock count (long integer) 0 = midnight and increments until a 24 hour equiv. | |
| 0470 | 1 | not 0 = passed midnight | |
| 0471 | 1 | if bit 7 is set, ctrl-break was pressed | |
| 0472 | 2 | set to 1234h after initial power up telling a warm boot to skip mem check | |
| 0474 | 1 | Fixed disk last operation status (except ESDI drives) | |
| 0475 | 1 | Number of fixed disk drives | |
| 0476 | 1 | Fixed disk control byte | |
| 0477 | 1 | Fixed disk I/O port offset (XT only) | |
| 0478 | 4 | contains a time-out value for the respective parallel ports | |
| 047C | 4 | contains a time-out value for the respective RS-232 serial ports | |
| 0480 | 2 | points at the start of the keyboard buffer area | |
| 0482 | 2 | points at the end of the keyboard buffer area | |
| 0484 | 1 | this value is one less than the number of character rows displayed on the screen | |
| 0485 | 2 | height, in scan lines, of characters on the screen | |
| 0487 | 4 | Amount of video RAM available | |
| Initial setting of the EGA configuration switches, as well as other misc video status info. | |||
| 048B | 11 | control and status information regarding the floppy and hard disk drives | |
| 0496 | 2 | data area used to support the 101-key keyboard | |
| 0498 | 9 | real time clock control | |
| 04A1 | 1 | LANA DMA channel flags | |
| 04A2 | 2 | Status of LANA 0,1 | |
| 04A4 | 4 | Saved hard disk interrupt vector | |
| 04A8 | 4 | (EGA and VGA) contain segmented address of a table of video parameters and overrides | |
| 04AC | 8 | (reserved) | |
| 04B4 | 1 | Keyboard NMI control flags (convertible) | |
| 04B5 | 4 | Keyboard Break pending flags (convertible) | |
| 04B9 | 1 | Port 60h single byte Queue (convertible) | |
| 04BA | 1 | Scan code for last key (convertible) | |
| 04BB | 1 | NMI Buffer head pointer (convertible) | |
| 04BC | 1 | NMI Buffer tail pointer (convertible) | |
| 04BD | 16 | NMI scan code Buffer (convertible) | |
| 04CD | 1 | unknown | |
| 04CE | 2 | Day Counter (convertible) | |
| 04D0 | 16 | unknown | |
| 04F0 | 16 | intra-application communications area (ICA) | |
| ie: known area where one app can communicate with another app. | |||
| (not used much because of its small size) | |||
| Remember that 0000:0500h = 0050:0000h | |||
| 0500 | 1 | status of a print-screen operation | |
| 00h - OK | |||
| 01h - a print-screen operation currently in progress | |||
| FFh - error occurred during a print-screen operation | |||
| 0501 | 3 | Used by BASIC | |
| 0504 | 1 | if only 1 floppy installed, this byte indicates which drive letter it is using (a or b) | |
| 00h - acting as drive a: | |||
| 01h - acting as drive b: | |||
| 0505 | 10 | POST work area | |
| 050F | 1 | BASIC shell flag (2 if currently in shell) | |
| 0510 | 2 | holds ROM Basics default DS value (DO NOT CHANGE) | |
| 0512 | 4 | segment offset address of Basic's clock-tick interrupt handler | |
| 0516 | 4 | address for ROM Basic's Break-key-handling routine | |
| 051A | 4 | address for ROM Basic's floppy disk error-handling routine | |
| 0520 | 2 | DOS Dynamic Storage ??? | |
| 0522 | 14 | DOS Diskette Initialization Table ??? | |
| 0530 | 4 | MODE command ??? | |
| 0534 | 206 | unknown | |
| Remember that 0000:0600h = 0060:0000h | |||
| 0600 | 256 | scratch space used during DOS 2+ boot up | |
| Remember that 0000:0700h = 0070:0000h | |||
| DOS 2+ | |||
| 0700 | 256 | IO.SYS LOAD ADDRESS | |
| DOS 5+ | |||
| 0700 | 25 | ORIGINAL INTERRUPT VECTORS 10h,13h,15h,19h,1Bh | |
| Each value is stored as a BYTE for the interrupt number followed by a DWORD for the vector | |||
| 0719 | 231 | unknown | |
| Remember that 0000:0800h = 0080:0000h | |||
| 0800 | 102 | 80286 CPU - LOADALL WORKSPACE | |
| On the 80286 (unlike 80386), the state buffer from which the LOADALL instruction loads all internal registers is hardwired to physical address 000800h Several versions 3.x of MS-DOS leave an empty space at offset 100h in IO.SYS (which is loaded at 0070h:0000h) so that HIMEM.SYS can use LOADALL on 80286 machines without having to save/restore the area of memory that LOADALL uses | |||
| 0866 | 154 | unknown | |
| Remember that 0000:0900h = 0090:0000h | |||
| 0900 | 653,056 | DOS (COMMAND.COM, etc., User Memory) | |
| 9FC0h:0000h - If your system has Extended BIOS Data Area (see 0040:000Eh and :0013h above) | |||
| 0000 | 1 | Size of Extended BIOS Data Area in 1k blocks | |
| 0002 | 21 | unknown | |
| 0017 | 1 | Number of POST error entries | |
| 0018 | 10 | Error Log for above | |
| 0022 | 4 | Mouse Driver Device Far Call | |
| 0026 | 1 | Mouse Flags 1 | |
| 0027 | 1 | Mouse Flags 2 | |
| 0028 | 8 | Mouse Data | |
| 0030 | 9 | unknown | |
| 0039 | 2 | Watch Dog Timer | |
| 003B | 2 | unknown | |
| 003D | 16 | Hard Disk 0 Parameter Table | |
| 004D | 16 | Hard Disk 1 Parameter Table | |
| 005D | 11 | unknown | |
| 0068 | 1 | Cache Control | |
| 0069 | 5 | unknown | |
| 006E | 1 | Repeat Rate of Keyboard | |
| 006F | 1 | Delay until Keyboard repeats | |
| 0070 | 1 | Number of Hard Drives attached | |
| 0071 | 1 | DMA channel Hard Drive | |
| 0072 | 1 | Hard Drive interrupt status | |
| 0073 | 1 | Hard Drive operation flags | |
| 0074 | 4 | Old interrupt 76h vector pointer | |
| 0078 | 1 | Hard Drive DMA type | |
| 0079 | 1 | (Hard Drive) status of last operation | |
| 007A | 1 | (Hard Drive) timeout value | |
| 007B | 3 | unknown | |
| 007E | 16 | Hard Drive controller return status words | |
| 008E | 89 | unknown | |
| 00E7 | 1 | Diskette Drive type | |
| 00E8 | 4 | unknown | |
| 00EC | 1 | Hard Drive Parameters Loaded | |
| 00ED | 1 | unknown | |
| 00EE | 1 | CPU Family ID | |
| 00EF | 1 | CPU Stepping | |
| 00F0 | 39 | unknown | |
| 0117 | 2 | Keyboard ID | |
| 0119 | 1 | unknown | |
| 011A | 1 | Non-BIOS Interrupt 18h Flag | |
| 011B | 2 | unknown | |
| 011D | 4 | User Interrupt 18h Far Pointer | |
| 0121 | 735 | unknown | |
| A000:0000h | |||
| 0000 | 65,536 | Graphics Video memory (EGA and above) | |
| B000:0000h | |||
| 0000 | 32,768 | Graphics area for EGA and up | |
| 8000 | 4096 | Page 0 - 80x25 16 color and mono (B800:0000) | |
| 9000 | 4096 | Page 1 - 80x25 16 color and mono (B800:1000) | |
| A000 | 4096 | Page 2 - 80x25 16 color and mono (B800:2000) | |
| B000 | 4096 | Page 3 - 80x25 16 color and mono (B800:3000) | |
| C000 | 4096 | Page 4 - 80x25 16 color and mono (B800:4000) | |
| D000 | 4096 | Page 5 - 80x25 16 color and mono (B800:5000) | |
| E000 | 4096 | Page 6 - 80x25 16 color and mono (B800:6000) | |
| F000 | 4096 | Page 7 - 80x25 16 color and mono (B800:7000) | |
| C000:0000h | |||
| 0000 | 65,536 | additional ROM-BIOS & video memory | |
| D000:0000h | |||
| 0000 | 65,536 | ROM cartridges | |
| E000:0000h | |||
| 0000 | 65,536 | ROM cartridges | |
| F000:0000h | |||
| 0000 | 32,768 | IBM PC ROM BASIC | |
| E000 | 8,192 | ORIGINAL IBM PC ROM BIOS | |
| FFF0 | 5 | RESET JUMP | |
| FFF5 | 8 | ASCII BIOS DATE | |
| FFFD | 1 | OFTEN USED TO ENSURE CORRECT BIOS CHECKSUM | |
| FFFE | 1 | MACHINE TYPE CODE | |
| FFFF | 1 | unknown | |
| Total memory | |||
| 0000:0000—F000:FFFF | 1,048,576 | 16 x 65,536 = 1 meg | |
-------------------------- Figures ---------------------------
****************************************************************
bit
FEDCBA98 76543210 Described
XX...... ........ Number of printers installed
..X..... ........ (reserved)
...X.... ........ set if game adapter installed
....XXX. ........ Number of RS-232 serial ports
.......X ........ (reserved)
........ XX...... +1 = number of floppy drives (see bit 0) 00 - 1 drive, 01 - 2 drives, etc.
........ ..XX.... Initial video mode
01 - 40 column color
10 - 80 column color
11 - monochrome
00 - none of the above
........ ....XX.. For PC w/64k motherboard
Amount of system board RAM
00 - 16k,01 - 32k, 10 - 48k, 11 - 64k
For PC/AT: not used
For PS/2:
Bit 3 not used
Bit 2 - pointing device installed
........ ......X. set if math co processor installed
........ .......X set if any floppy drives installed
if set, see bits 6 and 7
****************************************************************
FIGURE 1: equipment list word at 0000:0410h
****************************************************************
bits
76543210 Described
0000:0417h
X....... insert state: (1 = active, 0 = inactive)
.X...... caps lock:
..X..... num lock:
...X.... scroll lock:
....X... set if alt pressed
.....X.. set if ctrl pressed
......X. set if left shift pressed
.......X set if right shift pressed
0000:0418h
X....... set if insert pressed
.X...... set if caps lock pressed
..X..... set if num lock pressed
...X.... set if scroll lock pressed
....X... hold state active (ctrl-num lock or pause pressed)
.....X.. system request pressed
......X. left alt pressed
.......X right alt pressed
****************************************************************
FIGURE 2: Keyboard status bits at 0000:0417h, lo order bits are set
only while a corresponding key is pressed
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