3.1 Memory accessing methods

GNU Smalltalk provides methods for directly accessing real memory. You may access memory either as individual bytes, or as 32 bit words. You may read the contents of memory, or write to it. You may also find out the size and alignment of scalar C types, and determine the real memory address of an object or the real memory address of the OOP table that points to a given object, by using messages to the Memory class, described below.

Method: Memory bigEndian

Answers true on machine architectures where the most significant byte of a 32 bit integer has the lowest address (e.g. 68000 and Sparc), and false on architectures where the least significant byte occurs at the lowest address (e.g. Intel and VAX).

Variable: C*Size

Variable: C*Alignment

The CIntSize, CLongSize, CShortSize, CFloatSize, CDoubleSize, CPtrSize, CDoubleAlignment globals are provided by the VM as part of the Smalltalk dictionary, and are there for compatibility with old versions of GNU Smalltalk. However you should not use them and, instead, send messages like CInt sizeof or CDouble alignof.

Method: Object asOop

Returns the index of the OOP for anObject. This index is immume from garbage collection and is the same value used by default as an hash value for anObject (it is returned by Object's implementation of hash and identityHash).

Method: Integer asObject

Converts the given OOP index (not address) back to an object. Fails if no object is associated to the given index.

Method: Integer asObjectNoFail

Converts the given OOP index (not address) back to an object. Returns nil if no object is associated to the given index.

Other methods in ByteArray and Memory allow to read various C types (doubleAt:, ucharAt:, etc.). For examples of using asOop and asObject, look at the Blox source code in `blox/tk/BloxBasic.st'.

Another interesting class is ObjectMemory. This provides a few methods that enable one to tune the virtual machine"s usage of memory; many methods that in the past were instance methods of Smalltalk or class methods of Memory are now class methods of ObjectMemory. In addition, and that's what the rest of this section is about, the virtual machines signals events to its dependants exactly through this class.

The events that can be received are

returnFromSnapshot

This is sent every time an image is restarted, and substitutes the concept of an init block that was present in previous versions.

aboutToQuit

This is sent just before the interpreter is exiting, either because ObjectMemory quit was sent or because the specified files were all filed in. Exiting from within this event might cause an infinite loop, so be careful.

aboutToSnapshot

This is sent just before an image file is created. Exiting from within this event will leave any preexisting image untouched.

finishedSnapshot

This is sent just after an image file is created. Exiting from within this event will not make the image unusable.