INT 21 u - LAN Manager Enhanced DOS - DosPeekMailslot
AX = 5F51h
BX = handle
ES:DI -> buffer
Return: CF clear if successful
AX = bytes read
CX = next item's size
DX = next item's priority
CF set on error
AX = error code
SeeAlso: AX=5F35h,AX=5F4Fh,AX=5F50h,AX=5F52h