INT 21 - Turbo Debug HARDWARE BREAKPOINTS - READ STATUS BLOCK
AH = 3Fh
BX = handle for character device "TDHDEBUG"
CX = number of bytes to read
DS:DX -> buffer for status block (see below)
Return: CF clear if successful
AX = number of bytes actually read
CF set on error
AX = error code (05h,06h) (see AH=59h)

Format of status block:
Offset  Size    Description
00h    BYTE    status of command
00h successful
01h invalid handle
02h no more breakpoints available
03h hardware does not support specified breakpoint type
04h previous command prevents execution
05h debugger hardware not found
06h hardware failure
07h invalid command
08h driver not initialized yet
FEh recursive entry (hardware breakpoint inside hw bp handler)
---status for command 01h---
01h    WORD    device driver interface version number (currently 1)
03h    WORD    device driver software version
05h    BYTE    maximum simultaneous hardware breakpoints
06h    BYTE    configuration bits
bit 0: CPU and DMA accesses are distinct
1: can detect DMA transfers
2: supports data mask
3: hardware pass counter on breakpoints
4: can match on data as well as addresses
07h    BYTE    supported breakpoint types
bit 0: memory read
1: memory write
2: memory read/write
3: I/O read
4: I/O write
5: I/O read/write
6: instruction fetch
08h    WORD    supported addressing match modes
bit 0: any address
1: equal to test value
2: not equal
3: above test value
4: below test value
5: below or equal
6: above or equal
7: within range
8: outside range
0Ah    WORD    supported data matches
bit 0: any data
1: equal to test value
2: not equal
3: above test value
4: below test value
5: below or equal
6: above or equal
7: within range
8: outside range
0Ch    BYTE    maximum data match length (01h, 02h, or 04h)
0Dh    WORD    size of onboard memory (in K)
0Fh    WORD    maximum number of trace-back events
11h    WORD    hardware breakpoint enable byte address segment (0000h if not
supported)
---status for command 04h---
01h    BYTE    handle to use when referring to the just-set breakpoint

See also: 2140

213F - Turbo Debug HARDWARE BREAKPOINTS - READ STATUS BLOCK