Int 2F Fn 0802  U - Driver.sys Support - Execute Device Driver Request  (Cont.)

       Note:   the queue size field must be set before calling
       0Dh "QueueWrite" put a character in the queue
       DS:BX -> character queue (see #2287)
       AL = character to append to end of queue

       Return: ZF set if queue is full
           ZF clear if character stored
       0Eh "QueueRead" get a character from the queue
       DS:BX -> character queue (see #2287)

       Return: ZF set if queue is empty
           ZF clear if characters in queue
               AL = first character in queue
       10h "GetDOSVar" return pointer to DOS variable
       AL = index of variable
           03h current process ID
       BX = index into variable if AL specifies an array
       CX = expected length of variable

       Return: CF clear if successful
               DX:AX -> variable
           CF set on error
               AX,DX destroyed
           BX,CX destroyed

       Note:   the variables may not be modified
       14h "Yield" yield CPU if higher-priority task ready to run

       Return: FLAGS destroyed
       1Bh "CritEnter" begin system critical section
       DS:BX -> semaphore (6 BYTEs, initialized to zero)

       Return: AX,BX,CX,DX destroyed
       1Ch "CritLeave" end system critical section
       DS:BX -> semaphore (6 BYTEs, initialized to zero)

       Return: AX,BX,CX,DX destroyed

       Note:   must be called in the context of the process which
             called CritEnter on the semaphore

Note:  the DWORD pointing at the request queue must be allocated by the driver
     and initialized to 0000h:0000h.  It always points at the next request
     to be executed

Format of European MS-DOS 4.0 character queue:
Offset Size    Description (Table 2287)
 00h   WORD    size of queue in bytes
 02h   WORD    index of next character out
 04h   WORD    count of characters in the queue
 06h  N BYTEs  queue buffer