Specifications

CHAPTER 4

Software Support

74 Passing Messages

After the message has been sent, the msgResult ﬁeld is set to 0 (no error) or -3

(MsgTimeout). The msgActCount ﬁeld contains the number of bytes actually sent. If

you have speciﬁed a completion routine, it is then called.

Sending a Message From the Mac OS 4

To send a message on the Mac OS, build a MsgPBlk data structure and then pass the

pointer to the interface driver in an rsSendMessage control call.

The MsgPBlk data structure for applications on the Mac OS has the following format:

Parameter block

—> indicates input to the driver

<— indicates output from the driver

Your completion routine is called at deferred time and can use registers D0–D2 and

A0–A2. You must save all other registers. Upon return, A0 contains a pointer to the

MsgPBlk structure.

MsgPBlk RECORD 0

msgQLink DS.1 1 ; Next queue element

msgQType DS.w 1 ; Queue flag

msgCmd DS.w 1 ; The message type or command

msgParam1 DS.1 1 ; Message parameter 1

msgParam2 DS.1 1 ; Message parameter 2

msgBuffer DS.1 1 ; Pointer to the message data buffer

msgReqCount DS.1 1 ; Requested data length

msgActCount DS.1 1 ; Actual data length

msgCompletion DS.1 1 ; Pointer to completion routine or

NULL

msgResult DS.w 1 ; The result of any message operation

msgFlags DS.w 1 ; Message flags (Swap, and Shared);

set to zero!

msgUserData DS.1 1 ; For the caller’s use

MsgPBlkcSize Equ * ; Size of record

ENDR

—> ioCompletion long Pointer to the completion routine.

<— ioResult word Device driver’s result code.

—> ioRefNum word Device driver’s reference number.

—> csCode word Equals rsSendMessage.

—> csParam+0 long Pointer to MsgBlk.