Xlat_API_Calc_Len

Xlat_API_Calc_Len  Segment, Offset, Calc_Proc_Addr
 

Copies a calculated number of bytes from the protected-mode address to the translation copy buffer, and converts the address into a V86-mode address.

• No return value.

Segment

Register containing the selector for the protected-mode address.

Offset

Register containing the offset for the protected-mode address.

Calc_Proc_Addr

Name of the callback procedure that calculates the length, in bytes, of the data to copy. The V86MMGR_Xlat_API service calls the callback procedure with the FS:ESI register pair pointing to the buffer. The callback procedure must return the number of bytes to copy in the ECX register, and must preserve all other registers.

The V86MMGR_Xlat_API service fails if there is not enough room in the translation buffer to copy the specified number of bytes.

For example, Buffered Keyboard Input (Interrupt 21h Function 0Ah) can have a buffer size from 3 to 257 bytes; the size depends on the value of the first byte in the buffer. The following script and callback procedure translates this function:

VxD_DATA_SEG
Buff_Keyboard_Input_API:
    Xlat_API_Calc_Len ds, dx, Calc_Input_Buff_Size
    Xlat_API_Exec_Int 21h
VxD_DATA_ENDS

VxD_CODE_SEG
BeginProc Calc_Input_Buff_Size
    movzx  ecx, BYTE PTR fs:[esi]
    add    ecx, 2
    ret
EndProc Calc_Input_Buff_Size
VxD_CODE_ENDS