AT & XT Memory Address ---------------------- 10000h 1024k -------------------------------------------------- FC00h 1008k F800h 992k ROM BIOS F400h 976k F000h 960k -------------------------------------------------- EC00h 944k EC00h 944k E800h 928k Extra ROM for AT Free in XT E400h 912k E000h 896k --------------------- DC00h 880k -------------------------------------------------- D800h 864k Free in AT D400h 848k --------------------- XT LIM Data Area D000h 832k CC00h 816k LIM Data Area ----------------------------------- XT Hard disk ROM C800h 800k -------------------------------------------------- C400h 784k -------------------------------------------------- EGA & VGA ROM BIOS C000h 768k -------------------------------------------------- BC00h 752k -------------------------------------------------- CGA Video B800h 736k -------------------------------------------------- B400h 720k -------------------------------------------------- MDA Video B000h 704k -------------------------------------------------- AC00h 688k A800h 672k EGA & VGA Video RAM GRAPHICS A400h 656k A000h 640k -------------------------------------------------- =============================================================================== 0000 0000 - 0009 FFFF 640k BASE MEMORY AREA 000A 0000 - 000A FFFF 64k VIDEO RAM of EGA VGA 8514A in GRAPHICS MODE 000B 0000 - 000B 7FFF 32k VIDEO RAM of 1.CGA 2.HERCULES in GRAPHICS 3.EGA VGA 8514A in TEXT MODE 000B 8000 - 000B FFFF 32k VIDEO RAM of 1.MDA 2.HERCULES TEXT & GRAPHICS 000C 0000 - 000C 3FFF 16k ROM BIOS of EGA VGA 8514A 000C 4000 - 000C 7FFF 16k ROM BIOS of VGA 8514A 000C 8000 - 000C BFFF 16k ROM BIOS of XT HARD DISK CONTROLLER 000C C000 - 000D FFFF 80k Open for EXPANSION CARD 000E 0000 - 000E FFFF 64k IBM ROM BASIC 000F 0000 - 000F 7FFF 32k SYSTEM ROM BIOS for AT 000F 8000 - 000F DFFF 24k SYSTEM ROM BIOS 000F E000 - 000F FFFF 8k SYSTEM ROM BIOS 0010 0000 - 00FF 0000 15296k Extended memory area =============================================================================== DMA (8237) Channel Usage ------------------------ DMA 0 Available on AT, DRAM refresh on XT DMA 1 Available on I/O slot DMA 2 Floppy Diskette controller DMA 3 Available on I/O slot --------------------------------------+ DMA 4 Cascade for channel 0 - 3 | DMA 5 Available on I/O slot | NOT AVAILABLE FOR XT's DMA 6 Available on I/O slot | DMA 7 Available on I/O slot | --------------------------------------+ =============================================================================== Timer/Counter Channels (8253/8254) ---------------------------------- TC 0 System timer tick,generate IRQ 0 TC 1 DRAM refresh request, triggers refresh cycle for AT TC 1 DRAM refresh request, triggers DMA DRQ 0 for XT TC 2 Speaker tone generator (Programmable) =============================================================================== Hardware Interrupt Channels (8259) ---------------------------------- IRQ 0 Timer channel IRQ 1 Keyboard IRQ 2 Cascade IRQ8-IRQ15 (Becomes IRQ 9) (Available on XT's) IRQ 3 Second & Fourth RS-232C port IRQ 4 First & Third RS-232C port IRQ 5 Second parallel port (Hard disk controller for XT's) IRQ 6 Floppy disk controller IRQ 7 First parallel port -------------------------------------------+ IRQ 8 Real time clock | IRQ 9 BIOS redirected to INT 0A(H) | IRQ 10 Available | IRQ 11 Available | IRQ 12 Available | NOT AVAILABLE FOR XT's IRQ 13 Coprocessor (80287,80387) exeption | IRQ 14 Hard disk controller for AT | IRQ 15 Available | -------------------------------------------+ =============================================================================== I/O Space --------- 000-00F First DMA Controller (8237) 010-01F Available for expansion adapter 020-021 First interrupt controller (8259A) 022-03F Reserved by IBM, seldom used 040-047 Timer/counter (8253 for XT / 8254 for AT) 048-04F Reserved by IBM, seldom used 050-05F Reserved by IBM, seldom used 060-064 8255 ports for XT 8042 controller for AT 065-06F Reserved by IBM, seldom used 070-071 Real time clock for AT 072-07F Reserved by IBM, seldom used 080-08F DMA page registers 090-09F Reserved by IBM, seldom used 0A0-0A1 NMI mask register for XT Second interrupt controller for AT 0A2-0Af Reserved by IBM, seldom used 0B0-0BF Reserved by IBM, seldom used 0C0-0CF Second DMA controller for AT 0D0-0DF Reserved by IBM, seldom used 0E0-0EF Available 0F0-0FF Math coprocessor for AT (80287 or 80387) 100-1EF Available 1F0-1FF Hard disk controller for AT 200-207 Games ports adapter 208-20F Available 210-21F Expansion unit for XT 220-26F Available 270-277 Reserved by IBM, seldom used 278-27F Second parallel printer port 280-2AF Available 2B0-2DF EGA adapter alternate I/O address IBM 3270 emulator use's 2D0-2DF 2E8-2EF Fourth Serial Port 2E0-2EF Available 2F0-2F7 Reserved by IBM, seldom used 2F8-2FF Second RS-232C adapter 300-31F Available 320-32F Hard disk controller for XT 330-36F Available 370-377 Reserved by IBM, seldom used 378-37F First parallel printer port 380-38F SDLC adapter 390-3AF Available 3B0-3BB Reserved by IBM, seldom used 3BC-3BF Monochrome display adapter 3C0-3CF EGA/VGA 3D0-3DF CGA 3E0-3EF Available 3F0-3F7 Floppy disk controller 3F8-3FF First RS-232C adapter Description PC/XT AT PCjr ------------------------------------------------------------------------------- DMA Controller 1 000h-00Fh 000h-01Fh ... Interrupt Controller 1 020h-021h 020h-03Fh 020h-027h Timer 040h-043h 040h-05Fh 040h-047h PPI (Keyboard) 060h-063h ... 060h-067h AT Keyboard ... 060h-06Fh ... DMA Page Register 080h-083h 080h-09Fh ... NMI Mask Register 0Ah 070h-07Fh 0A0h-0A7h AT Interrupt Controller 2 ... 0A0h-0BFh ... PCjr Sound Generator ... ... 0C0h-0C7h AT DMA Controller 2 ... 0C0h-0DFh ... AT Clear/Reset 80287 ... 0F0h-0F1h ... AT 80287 Math Coprocessor ... 0F8h-0FFh ... Joystick 200h-20Fh 200h-207h 200h-207h PC/XT Expansion Unit 210h-217h ... ... AT Secondary Parallel Printer ... 278h-27Fh ... Fourth Serial Port 2E8h-2EFh Secondary Serial Port 2F8h-2FFh 2F8h-2FFh ... Prototype Card 300h-31Fh 300h-31Fh ... Fixed Disk 320h-32Fh 1F0h-1F8h ... Primary Parallel Printer 378h-37Fh 378h-37Fh ... PC/XT SDLC (Mainframe Comm) 380h-38Fh ... ... AT Secondary Bisynch Comm ... 380h-38Fh ... AT Primary Bisynch Comm ... 3A0h-3AFh ... Monochrome Graphics Adapter 3B0h-3BFh 3B0h-3BFh ... Color Graphics Adapter 3D0h-3DFh 3D0h-3DFh ... Third Serial Port 3E8h-3EFh Diskette Controller 3F0h-3F7h 3F0h-3F7h 0F0h-0FFh Primary Serial Port 3F8h-3FFh 3F8h-3FFh 2F8h-2FFh =============================================================================== Software Interrupt Vectors -------------------------- 00 Divide by Zero CPU 01 Single step CPU 02 Nonmaskable interrupt (NMI) Memory Parity/Math Coprocessor 03 Breakpoint CPU 04 Overflow CPU 05 Bound range exceeded CPU (80286 and up) 06 Invalid instruction code CPU 07 Coprocessor not present CPU 08 Timer tick interrupt IRQ 0 09 Keyboard interrupt IRQ 1 0A Cascade IRQ 08-0F for AT IRQ 2 0B Second RS-232C interrupt IRQ 3 0C First RS-232C interrupt IRQ 4 0D Second Parallel port Interrupt IRQ 5 Hard disk controller interrupt 0E Floppy diskette controller IRQ 6 0F First Parallel port interrupt IRQ 7 10 Video services routine BIOS support INT 10 11 Equipment inquiry service BIOS support INT 11 12 Memory size inquiry service BIOS support INT 12 13 Diskette I/O service BIOS support INT 13 14 RS-232C communication BIOS support INT 14 15 Cassette tape drive I/O BIOS support INT 15 Extended services 16 Keyboard buffer read/write BIOS support INT 16 17 Parallel printer service BIOS support INT 17 18 IBM BASIC Resident portion IBM ROM BASIC 19 Bootstrap service BIOS support INT 18 check 1A Time of day inquiry BIOS support INT 19 1B Keyboard break (Ctrl-Break) BIOS internal use 1C Timer tick counter routine BIOS internal use 1D Address of video parameters table BIOS internal use 1E Address of diskette parameters table BIOS internal use 1F Video graphics EGA,VGA,ROM BIOS 20 Program termination DOS supports INT 20 21 MS-DOS service entry point DOS supports INT 21 22 Program's terminate address DOS internal use 23 Ctrl-Break exit address DOS internal use 24 Fatal error handlers routine DOS internal use 25 Absolute disk read DOS supports INT 25 26 Absolute disk write DOS supports INT 26 27 Terminate and stay resident DOS supports INT 27 28 Available | 29 Available | 2A Available | 2B Available | INT HH 2C Available | 2D Available | 2E Available | 2F MS-DOS installable services DOS internal use 30-3F Reserved by MS-DOS 40 Hard disk I/O BIOS from INT 13 41 Address of 1st hard disk parameters BIOS internal use table 42 EGA VGA video services EGA,VGA,ROM BIOS 43 Address of EGA VGA parameters table EGA,VGA,ROM BIOS 44 Available 45 Available 46 Address of 2nd hard disk parameters BIOS internal use table 47 Available 48 Available 49 Available 50 Clock alarm service BIOS internal use 51-5A Available INT HH 5B Revector from INT19 BIOS internal use 5C IBM PC LAN network service NETBIOS entry point 5D-66 Available 67 LIM EMS services entry point EMS driver 68-6F Available 70 Real time clock interrupt IRQ 8 (AT only) 71 Redirected to INT 9 IRQ 9 (AT only) 72 Available on I/O slot IRQ 10 (AT only) 73 Available on I/O slot IRQ 11 (AT only) 74 Available on I/O slot IRQ 12 (AT only) 75 AT math coprocessor exception IRQ 13 (AT only) 76 AT Hard disk controller interrupt IRQ 14 (AT only) 77 Available on I/O slot IRQ 15 (AT only) 78-7F Available INT HH 80-F0 Reserved by IBM BASIC IBM BASIC, internal use F1-FF Available ===============================================================================