Ship documentation on bios data area

doc/bda.txt

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+The actual layout of the BDA is specific to the actual BIOS firmware running.
+This lists is focused on what a developer can reasonably rely on.
+
+0000 256 Interrupt vectors
+0300 256 bytes used as stack during POST
+0400 word: COM1 base I/O port
+0402 word: COM2
+0404 word: COM3
+0406 word: COM4
+0408 word: LPT1 base I/O port
+040A word: LPT2
+040C word: LPT3
+040E word: segment addr of the EBDA (rarely LPT4)
+0410 byte: equipment list flags (INT 11h, AL)
+     bit 0: IPL diskette installed
+     bit 1: math coprocessor present
+     bits 4-5: default VDU mode
+     bits 6-7: number of floppies minus one, 0 if none
+0411 byte: equipment list flags (INT 11h, AH)
+     bit 0: dma controller present
+     bits 1-3: number of serial ports
+     bit 4: game adapter present
+     bit 6-7: number of printer ports
+0413 word: memory size in kilobytes (INT 12h, AX)
+0417 byte: key toggle states
+     bit 0: right shift
+     bit 1: left shift
+     bit 2: ctrl
+     bit 3: alt
+     bit 4: scroll lock
+     bit 5: num lock
+     bit 6: caps lock
+     but 7: insert
+0418 byte: key pressed state (1 if currently held down)
+     bit 0: left ctrl key
+     bit 1: left alt key
+     bit 2: system key
+     bit 3: suspend key
+     bit 4: scroll lock
+     bit 5: num lock
+     bit 6: caps lock
+     bit 7: insert
+0419 byte: when typing a character by pressing the alt key and typing its code
+     into the numpad, this byte holds the current numeric value
+041A word: offset into the key ringbuffer, reading end
+041B word: offset into the key ringbuffer, writing end
+041E 16 words: key ringbuffer, holds place for 16 keypresses
+043E byte: drive restore flag
+043F byte: drive motor flag
+0440 byte: drive motor timeout counter in 1/18th seconds
+0441 byte: status of last disk operation
+0442 7 bytes: status buffer for FDC
+0449 byte: current VDU mode
+044A byte: number of visible display columns
+044C word: VDU display buffer size
+044E word: offset of current VDU page in buffer
+0450 8 words: 8 cursor positions for each VDU page (MSB=row, LSB=column)
+0460 byte: bottom scan line for cursor (dont write)
+0461 byte: top scan line for cursor (dont write)
+0462 byte: active display page number
+0463 word: VDU I/O address
+0465 byte: VDU mode control register
+0466 byte: VDU color select register
+0467 dword CS:IP for 80286 return from protected mode
+046C dword: time of the day in 1/18th second ticks
+0470 byte: rollover flag if above surpassed 24h (FFh = rollover)
+0471 byte: bit 7 is set on ctrl+numlock to indicate break condition
+0472 word: soft reset flag (set and JMP 0xFFFF:0)
+     1234: bypas memory tests and VDU setup
+     4321: preserve memory
+     5678: system suspend
+     9ABC: POST loop (on IBM convertible)
+0474 dword: reserved for hard disk bios
+0478 byte: LPT1 timeout in half seconds
+0479 byte: LPT2
+047A byte: LPT3
+047C byte: COM1 timeout in half seconds
+0480 word: key ringbuffer start (from 0x400)
+0482 word: key ringbuffer
+0500 byte: status of INT 5: print screen
+     00: no screen print in progress
+     01: in progress
+     FF: printer failure