Specifications

CHAPTER 4
Software
System Software 43
Because the Screen driver is opened at startup and is never closed, and because there is
nothing to read or write, the following calls are not supported:
■ CloseDriver
■ FSRead
■ FSWrite
■ KillIO
The following are calls supported by the Screen Driver.
Open 4
PBOpen (paramBlock: ParmBlkPtr; async: BOOLEAN) : OSErr_Open
Parameter block
When the system software opens the Screen driver at startup time, the Device Manager
creates a device control entry (DCE) and stores its handle in the proper unit table entry.
Subsequent Open calls merely return refNum and a result (noErr). Programs can use the
Open call to obtain the refNum value for use in control and status calls.
High-level call:
OpenDriver (name: Str255; VAR refNum: INTEGER) : OSErr
Control 4
PBControl (paramBlock: ParmBlkPtr; async: BOOLEAN) : OSErr_Control
Parameter block
→ ioCompletion Nil.
← ioResult noErr (zero).
→ ioNamePtr Pointer to the name .BCScreen.
← ioRefNum Driver’s reference number.
→ ioPermssn Must be fsCurPerm.
→ ioCompletion Nil.
← ioResult controlErr (–17) if csCode is unimplemented;
otherwise noErr (0).
→ ioRefNum Driver’s reference number.
→ csCode Identifies the call; described below.
→ csParam Depends on the call; described with the csCode values
where it is used (csParam is an array of 11 shorts).