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