Technical Q&A

HW 08 - Implementing read-modify-write on PCI (15-July-95)

Atomic Memory Operations
The Driver Services Library provides the following 32-, 16-, and 8-bit atomic memory operations for use by device drivers.

Boolean CompareAndSwap(long oldValue, long newValue, long *Value);

SInt32			IncrementAtomic( SInt32 *value );
SInt32			DecrementAtomic( SInt32 *value );
SInt32			AddAtomic( SInt32 amount, SInt32 *value );
UInt32		BitAndAtomic( UInt32 mask, UInt32 *value );
UInt32		BitOrAtomic( UInt32 mask, UInt32 *value );
UInt32		BitXorAtomic( UInt32 mask, UInt32 *value );
SInt8			 IncrementAtomic8( SInt8 *value );
SInt8			 DecrementAtomic8( SInt8 *value );
SInt8			 AddAtomic8( SInt32 amount, SInt8 *value );
UInt8			 BitAndAtomic8( UInt32 mask, UInt8 *value );
UInt8			 BitOrAtomic8( UInt32 mask, UInt8 *value );
UInt8			 BitXorAtomic8( UInt32 mask, UInt8 *value );
SInt16			IncrementAtomic16( SInt16 *value );
SInt16			DecrementAtomic16( SInt16 *value );
SInt16			AddAtomic16( SInt32 amount, SInt16 *value );
UInt16		BitAndAtomic16( UInt32 mask, UInt16 *value );
UInt16		BitOrAtomic16( UInt32 mask, UInt16 *value );
UInt16		BitXorAtomic16( UInt32 mask, UInt16 *value );

The atomic routines perform various operations on the memory address specified by value:

* IncrementAtomic increments the value by one and DecrementAtomic decrements it by one. These functions return the value as it was before the change.
* AddAtomic adds the specified amount to the value at the specified address and returns the result.
* BitAndAtomic performs a logical and operation between the bits of the specified mask and the value at the specified address, returning the result. Similarly, BitOrAtomic performs a logical or operation and BitXorAtomic performs a logical XOR operation.
* The CompareAndSwap routine compares the value at the specified address with oldValue. The value of newValue is written to the specified address only if oldValue and the value at the specified address are equal. CompareAndSwap returns true if newValue is written to the specified address; otherwise it returns false. A false return value does not imply that oldValue and the value at the specified address are not equal; it only implies that CompareAndSwap did not write newValue to the specified address.

These routines take logical address pointers and ensure that the operations are atomic with respect to all devices (for example, other processors, DMA engines) that participate in the coherency architecture of the Macintosh system.

TestAndSet		(UInt32			theBit
                  UInt8            *theAddress);
TestAndClear	(UInt32			theBit
                  UInt8			   *theAddress);

theBit	        The bit number in the range 0 through 7.
theAddress	    The address of the byte in which the bit is located.

TestAndSet and TestAndClear set and clear a single bit in a byte at a specified address.