Mac OS memory management
Encyclopedia
Historically, the Mac OS
used a form of memory management
that has fallen out of favour in modern systems. Criticism of this approach was one of the key areas addressed by the change to Mac OS X
.
The original problem for the engineers of the Macintosh
was how to make optimum use of the 128 KB of RAM
that the machine was equipped with. Since at that time the machine could only run one application
program at a time, and there was no fixed secondary storage, the engineers implemented a simple scheme which worked well with those particular constraints. However, that design choice did not scale well with the development of the machine, creating various difficulties for both programmers and users.
- that is, the repeated allocation and deallocation of memory through pointers leads to many small isolated areas of memory which cannot be used because they are too small, even though the total free memory may be sufficient to satisfy a particular request for memory. To solve this, Apple engineers used the concept of a relocatable handle
, a reference to memory which allowed the actual data referred to be moved without invalidating the handle. Apple's scheme was simple - a handle was simply a pointer into a (non relocatable) table of further pointers, which in turn pointed to the data.
If a memory request required compaction of memory, this was done and the table, called the master pointer block, was updated. The machine itself implemented two areas in the machine available for this scheme - the system heap (used for the OS), and the application heap.
As long as only one application at a time was run, the system worked well. Since the entire application heap was dissolved when the application quit, fragmentation was minimized.
However, in addressing the fragmentation problem, all other issues were overlooked. The system heap was not protected from errant applications, and this was frequently the cause of major system problems and crashes.
In addition, the handle-based approach also opened up a source of programming errors, where pointers to data within such relocatable blocks could not be guaranteed to remain valid across calls that might cause memory to move. In reality this was a problem in almost every system API
that existed. Thus the onus was on the programmer not to create such pointers, or at least manage them very carefully. Since many programmers were not generally familiar with this approach, early Mac programs suffered frequently from faults arising from this - faults that very often went undetected until long after shipment.
Palm OS
and 16-bit Windows use a similar scheme for memory management. However, the Palm and Windows versions make programmer error more difficult. For instance, in Mac OS to convert a handle to a pointer, a program just de-references the handle directly. However, if the handle is not locked, the pointer can become invalid quickly. Calls to lock and unlock handles are not balanced; ten calls to HLock are undone by a single call to HUnlock. In Palm OS and Windows, handles are opaque type and must be de-referenced with MemHandleLock on Palm OS or Global/LocalLock on Windows. When a Palm or Windows application is finished with a handle, it calls MemHandleUnlock or Global/LocalUnlock. Palm OS and Windows keeps a lock count for blocks; after three calls to MemHandleLock, a block will only become unlocked after three calls to MemHandleUnlock.
Addressing the problem of nested locks and unlocks can be straightforward (although tedious) by employing various methods, but these intrude upon the readability of the associated code block and require awareness and discipline on the part of the coder.
The amount of actual RAM allocated to each heap was set by a value coded into each application, set by the programmer. Invariably this value wasn't enough for particular kinds of work, so the value setting had to be exposed to the user to allow them to tweak the heap size to suit their own requirements. This exposure of a technical implementation detail was very much against the grain of the Mac user philosophy. Apart from exposing users to esoteric technicalities, it was inefficient, since an application would grab (unwillingly) all of its allotted RAM, even if it left most of it subsequently unused. Another application might be memory starved, but was unable to utilize the free memory "owned" by another application.
While an application could not beneficially utilize a sister application's heap, it could most certainly destroy it, typically by inadvertently writing to a nonsense address. An application treating a fragment of text or image, or an unassigned location as an address could easily overwrite the code or data of other applications or even the OS, leaving "lurkers" even after the program was exited. Such problems could be extremely difficult to analyze and correct.
Switcher became MultiFinder in System 4.2, which became the Process Manager in System 7
, and by then the scheme was utterly entrenched. Apple made some attempts to work around the obvious limitations – temporary memory was one, where an application could "borrow" free RAM that lay outside of its heap for short periods, but this was unpopular with programmers so it largely failed to solve the problem. There was also an early virtual memory
scheme, which attempted to solve the issue by making more memory available by paging unused portions to disk, but for most users with 68K Macintoshes, this did nothing but slow everything down without solving the memory problems themselves. By this time all machines had permanent hard disks and MMU
chips, and so were equipped to adopt a far better approach. For some reason, Apple never made this a priority until Mac OS X, even though several schemes were suggested by outside developers that would retain compatibility while solving the overall memory management problem. Third party replacements for the Mac OS memory manager, such as Optimem, showed it could be done.
. These Macintosh computers used the 68000
CPU, a 32-bit processor, but only had 24 physical address lines. The 24 lines allowed the processor to address up to 16 MB of memory (224 bytes), which was seen as a sufficient amount at the time. However, the RAM limit in the Macintosh design was 4 MB of RAM and 4 MB of ROM
, because of the structure of the memory map. This was fixed by changing the memory map with the Macintosh II
and the Macintosh Portable
, allowing up to 8 MB of RAM.
Because memory was a scarce resource, the authors of the Mac OS decided to take advantage of the unused byte in each address. The original Memory Manager (up until the advent of System 7) placed flags in the high 8 bits of each 32-bit pointer and handle
. Each address contained flags such as "locked", "purgeable", or "resource", which were stored in the master pointer table. When used as an actual address, these flags were masked off and ignored by the CPU.
While a good use of very limited RAM space, this design led to problems once Apple introduced the Macintosh II, which used the 32-bit Motorola 68020
CPU. The 68020 had 32 physical address lines and could address up to 4GB (232 bytes) of memory. The flags that the Memory Manager stored in the high byte of each pointer and handle were significant now, and could lead to addressing errors.
In theory, the architects of the Macintosh system software were free to change the "flags in the high byte" scheme to avoid this problem, and they did. For example, on the Macintosh II, HLock was rewritten to implement handle locking in a way other than flagging the high bits of handles. However, many Macintosh application programmers — and a great deal of the Macintosh system software code itself, even the code in the ROMs (until the IIci, which made the ROM 32-bit clean) – accessed the flags directly rather than using the APIs, such as HLock, which had been provided to manipulate them. By doing this they rendered their applications incompatible with true 32-bit addressing, and this became known as not being "32-bit clean".
In order to stop continual system crashes caused by this issue, System 6 and earlier running on a 68020 or a 68030 would force the CPU into 24-bit mode, and would only recognize and address the first 8 megabytes of RAM, an obvious flaw in machines whose hardware was wired to accept up to 128MB RAM – and whose product literature advertised this capability. With System 7, the Mac system software was finally made 32-bit clean, but there were still the problem of dirty ROMs. The problem was that the decision to use 24-bit or 32-bit addressing has to be made very early in the boot process, when the ROM routines initialized the Memory Manager to set up a basic Mac environment where NuBus ROMs and disk driver are loaded and executed. Older ROMs did not have this support and so was not possible to boot into 32-bit mode. Surprisingly, the first solution to this flaw was published by software utility company Connectix
, whose 1991 product MODE32
reinitialized the Memory Manager and repeated early parts of the Mac boot process, allowing the system to boot into 32-bit mode and enabling the use of all the RAM in the machine. Apple licensed the software from Connectix later in 1991 and distributed it for free. The Macintosh IIci
and later Motorola
based Macintosh computers had 32-bit clean ROMs. (Even later models were based on the 64-bit AIM
PowerPC
processor and using New World ROM
. The most recent models use 32-bit Intel
x86-686 or 64-bit Intel x86-64
processors, connected to an EFI
.)
However it was quite a while before applications were updated to remove all 24-bit dependencies, and System 7 provided a way to switch back to 24-bit mode if application incompatibilities were found. By the time of migration to the PowerPC and System 7.1.2, 32-bit cleanliness was mandatory for creating native applications.
languages for programming the Mac – first Object Pascal
, then later C++
– also caused problems for the memory model adopted. At first, it would seem natural that objects would be implemented via handles, to gain the advantage of being relocatable. However, these languages, as they were originally designed, used pointers for objects, which would lead to fragmentation issues. A solution, implemented by the THINK (later Symantec
) compiler
s, was to use Handles internally for objects, but use a pointer syntax to access them. This seemed a good idea at first, but soon deep problems emerged, since programmers could not tell whether they were dealing with a relocatable or fixed block, and so had no way to know whether to take on the task of locking objects or not. Needless to say this led to huge numbers of bugs and problems with these early object implementations. Later compilers did not attempt to do this, but used real pointers, often implementing their own memory allocation schemes to workaround the Mac OS memory model.
While the Mac OS memory model, with all its inherent problems, remained this way right through to Mac OS 9
, due to severe application compatibility constraints, the increasing availability of cheap RAM meant that by and large most users could upgrade their way out of a corner. The memory wasn't used efficiently, but it was abundant enough that the issue never became critical. This is perhaps ironic given that the purpose of the original design was to maximise the use of very limited amounts of memory. Mac OS X
finally does away with the whole scheme, implementing a modern sparse virtual memory scheme. A subset of the older memory model APIs still exist for compatibility as part of Carbon
, but map to the modern memory manager (a threadsafe malloc
implementation) underneath.
Apple recommends that Mac OS X code use malloc and free "almost exclusively".
Mac OS
Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface...
used a form of memory management
Memory management
Memory management is the act of managing computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and freeing it for reuse when no longer needed. This is critical to the computer system.Several...
that has fallen out of favour in modern systems. Criticism of this approach was one of the key areas addressed by the change to Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
.
The original problem for the engineers of the Macintosh
Macintosh
The Macintosh , or Mac, is a series of several lines of personal computers designed, developed, and marketed by Apple Inc. The first Macintosh was introduced by Apple's then-chairman Steve Jobs on January 24, 1984; it was the first commercially successful personal computer to feature a mouse and a...
was how to make optimum use of the 128 KB of RAM
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
that the machine was equipped with. Since at that time the machine could only run one application
Application software
Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...
program at a time, and there was no fixed secondary storage, the engineers implemented a simple scheme which worked well with those particular constraints. However, that design choice did not scale well with the development of the machine, creating various difficulties for both programmers and users.
Fragmentation
The primary concern of the original engineers appears to have been fragmentationFragmentation (computer)
In computer storage, fragmentation is a phenomenon in which storage space is used inefficiently, reducing storage capacity and in most cases reducing the performance. The term is also used to denote the wasted space itself....
- that is, the repeated allocation and deallocation of memory through pointers leads to many small isolated areas of memory which cannot be used because they are too small, even though the total free memory may be sufficient to satisfy a particular request for memory. To solve this, Apple engineers used the concept of a relocatable handle
Handle (computing)
In computer programming, a handle is a particular kind of smart pointer. Handles are used when application software references blocks of memory or objects managed by another system, such as a database or an operating system....
, a reference to memory which allowed the actual data referred to be moved without invalidating the handle. Apple's scheme was simple - a handle was simply a pointer into a (non relocatable) table of further pointers, which in turn pointed to the data.
If a memory request required compaction of memory, this was done and the table, called the master pointer block, was updated. The machine itself implemented two areas in the machine available for this scheme - the system heap (used for the OS), and the application heap.
As long as only one application at a time was run, the system worked well. Since the entire application heap was dissolved when the application quit, fragmentation was minimized.
However, in addressing the fragmentation problem, all other issues were overlooked. The system heap was not protected from errant applications, and this was frequently the cause of major system problems and crashes.
In addition, the handle-based approach also opened up a source of programming errors, where pointers to data within such relocatable blocks could not be guaranteed to remain valid across calls that might cause memory to move. In reality this was a problem in almost every system API
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
that existed. Thus the onus was on the programmer not to create such pointers, or at least manage them very carefully. Since many programmers were not generally familiar with this approach, early Mac programs suffered frequently from faults arising from this - faults that very often went undetected until long after shipment.
Palm OS
Palm OS
Palm OS is a mobile operating system initially developed by Palm, Inc., for personal digital assistants in 1996. Palm OS is designed for ease of use with a touchscreen-based graphical user interface. It is provided with a suite of basic applications for personal information management...
and 16-bit Windows use a similar scheme for memory management. However, the Palm and Windows versions make programmer error more difficult. For instance, in Mac OS to convert a handle to a pointer, a program just de-references the handle directly. However, if the handle is not locked, the pointer can become invalid quickly. Calls to lock and unlock handles are not balanced; ten calls to HLock are undone by a single call to HUnlock. In Palm OS and Windows, handles are opaque type and must be de-referenced with MemHandleLock on Palm OS or Global/LocalLock on Windows. When a Palm or Windows application is finished with a handle, it calls MemHandleUnlock or Global/LocalUnlock. Palm OS and Windows keeps a lock count for blocks; after three calls to MemHandleLock, a block will only become unlocked after three calls to MemHandleUnlock.
Addressing the problem of nested locks and unlocks can be straightforward (although tedious) by employing various methods, but these intrude upon the readability of the associated code block and require awareness and discipline on the part of the coder.
Memory leaks and stale references
Awareness and discipline are also necessary to avoid memory "leaks" (failure to deallocate within the scope of the allocation) and to avoid references to stale handles after release.Switcher
The situation worsened with the advent of Switcher, which was a way for the Mac to run multiple applications at once. This was a necessary step forward for users, who found the one-app-at-a-time approach very limiting. However, because Apple was now committed to its memory management model, as well as compatibility with existing applications, it was forced to adopt a scheme where each application was allocated its own heap from the available RAM.The amount of actual RAM allocated to each heap was set by a value coded into each application, set by the programmer. Invariably this value wasn't enough for particular kinds of work, so the value setting had to be exposed to the user to allow them to tweak the heap size to suit their own requirements. This exposure of a technical implementation detail was very much against the grain of the Mac user philosophy. Apart from exposing users to esoteric technicalities, it was inefficient, since an application would grab (unwillingly) all of its allotted RAM, even if it left most of it subsequently unused. Another application might be memory starved, but was unable to utilize the free memory "owned" by another application.
While an application could not beneficially utilize a sister application's heap, it could most certainly destroy it, typically by inadvertently writing to a nonsense address. An application treating a fragment of text or image, or an unassigned location as an address could easily overwrite the code or data of other applications or even the OS, leaving "lurkers" even after the program was exited. Such problems could be extremely difficult to analyze and correct.
Switcher became MultiFinder in System 4.2, which became the Process Manager in System 7
System 7
System 7 is the name of a Macintosh operating system introduced in 1991.System 7 may also refer to:* System 7 , a British dance/ambient band* System 7 , 1991 album* IBM System/7, a 1970s computer system...
, and by then the scheme was utterly entrenched. Apple made some attempts to work around the obvious limitations – temporary memory was one, where an application could "borrow" free RAM that lay outside of its heap for short periods, but this was unpopular with programmers so it largely failed to solve the problem. There was also an early virtual memory
Virtual memory
In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...
scheme, which attempted to solve the issue by making more memory available by paging unused portions to disk, but for most users with 68K Macintoshes, this did nothing but slow everything down without solving the memory problems themselves. By this time all machines had permanent hard disks and MMU
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
chips, and so were equipped to adopt a far better approach. For some reason, Apple never made this a priority until Mac OS X, even though several schemes were suggested by outside developers that would retain compatibility while solving the overall memory management problem. Third party replacements for the Mac OS memory manager, such as Optimem, showed it could be done.
32-bit clean
Originally the Macintosh had 128 kB of RAM, with a limit of 512 kB. This was increased to 4 MB upon the introduction of the Macintosh PlusMacintosh Plus
The Macintosh Plus computer was the third model in the Macintosh line, introduced on January 16, 1986, two years after the original Macintosh and a little more than a year after the Macintosh 512K, with a price tag of US$2599...
. These Macintosh computers used the 68000
Motorola 68000
The Motorola 68000 is a 16/32-bit CISC microprocessor core designed and marketed by Freescale Semiconductor...
CPU, a 32-bit processor, but only had 24 physical address lines. The 24 lines allowed the processor to address up to 16 MB of memory (224 bytes), which was seen as a sufficient amount at the time. However, the RAM limit in the Macintosh design was 4 MB of RAM and 4 MB of ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
, because of the structure of the memory map. This was fixed by changing the memory map with the Macintosh II
Macintosh II
The Apple Macintosh II was the first personal computer model of the Macintosh II series in the Apple Macintosh line and the first Macintosh to support a color display.- History :...
and the Macintosh Portable
Macintosh Portable
The Macintosh Portable was Apple Inc.'s first attempt at making a battery-powered portable Macintosh personal computer that held the power of a desktop Macintosh...
, allowing up to 8 MB of RAM.
Because memory was a scarce resource, the authors of the Mac OS decided to take advantage of the unused byte in each address. The original Memory Manager (up until the advent of System 7) placed flags in the high 8 bits of each 32-bit pointer and handle
Handle (computing)
In computer programming, a handle is a particular kind of smart pointer. Handles are used when application software references blocks of memory or objects managed by another system, such as a database or an operating system....
. Each address contained flags such as "locked", "purgeable", or "resource", which were stored in the master pointer table. When used as an actual address, these flags were masked off and ignored by the CPU.
While a good use of very limited RAM space, this design led to problems once Apple introduced the Macintosh II, which used the 32-bit Motorola 68020
Motorola 68020
The Motorola 68020 is a 32-bit microprocessor from Motorola, released in 1984. It is the successor to the Motorola 68010 and is succeeded by the Motorola 68030...
CPU. The 68020 had 32 physical address lines and could address up to 4GB (232 bytes) of memory. The flags that the Memory Manager stored in the high byte of each pointer and handle were significant now, and could lead to addressing errors.
In theory, the architects of the Macintosh system software were free to change the "flags in the high byte" scheme to avoid this problem, and they did. For example, on the Macintosh II, HLock was rewritten to implement handle locking in a way other than flagging the high bits of handles. However, many Macintosh application programmers — and a great deal of the Macintosh system software code itself, even the code in the ROMs (until the IIci, which made the ROM 32-bit clean) – accessed the flags directly rather than using the APIs, such as HLock, which had been provided to manipulate them. By doing this they rendered their applications incompatible with true 32-bit addressing, and this became known as not being "32-bit clean".
In order to stop continual system crashes caused by this issue, System 6 and earlier running on a 68020 or a 68030 would force the CPU into 24-bit mode, and would only recognize and address the first 8 megabytes of RAM, an obvious flaw in machines whose hardware was wired to accept up to 128MB RAM – and whose product literature advertised this capability. With System 7, the Mac system software was finally made 32-bit clean, but there were still the problem of dirty ROMs. The problem was that the decision to use 24-bit or 32-bit addressing has to be made very early in the boot process, when the ROM routines initialized the Memory Manager to set up a basic Mac environment where NuBus ROMs and disk driver are loaded and executed. Older ROMs did not have this support and so was not possible to boot into 32-bit mode. Surprisingly, the first solution to this flaw was published by software utility company Connectix
Connectix
Connectix Corporation was a software and hardware company, noted for having released innovative products that were either made obsolete as Apple Computer incorporated the ideas into system software, or were sold to other companies once they become popular...
, whose 1991 product MODE32
MODE32
MODE32 is a software product originally developed by Connectix for certain models of the Apple Macintosh. It was published in 1991 and originally cost $169, however on September 5, 1991, the software was made available free to customers under licensing terms with Apple Computer.-Overview:MODE32...
reinitialized the Memory Manager and repeated early parts of the Mac boot process, allowing the system to boot into 32-bit mode and enabling the use of all the RAM in the machine. Apple licensed the software from Connectix later in 1991 and distributed it for free. The Macintosh IIci
Macintosh IIci
The Apple Macintosh IIci was an improvement on the Macintosh IIcx. Sharing the same compact case design with three expansion slots, the IIci improved upon the IIcx's 16 MHz Motorola 68030 CPU and 68882 FPU, replacing them with 25 MHz versions of these chips. The IIci came with either a 40 or...
and later Motorola
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
based Macintosh computers had 32-bit clean ROMs. (Even later models were based on the 64-bit AIM
AIM alliance
The AIM alliance was an alliance formed on October 2, 1991, between Apple Inc. , IBM, and Motorola to create a new computing standard based on the PowerPC architecture. The stated goal of the alliance was to challenge the dominant Wintel computing platform with a new computer design and a...
PowerPC
PowerPC
PowerPC is a RISC architecture created by the 1991 Apple–IBM–Motorola alliance, known as AIM...
processor and using New World ROM
New World ROM
New World ROM computers are Macintosh models that do not use a Macintosh Toolbox ROM on the logic board. Due to Mac OS X not requiring the availability of the Toolbox, this allowed ROM sizes to shrink dramatically , and facilitated the use of Flash memory for system firmware instead of the now more...
. The most recent models use 32-bit Intel
Intel Corporation
Intel Corporation is an American multinational semiconductor chip maker corporation headquartered in Santa Clara, California, United States and the world's largest semiconductor chip maker, based on revenue. It is the inventor of the x86 series of microprocessors, the processors found in most...
x86-686 or 64-bit Intel x86-64
X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
processors, connected to an EFI
Extensible Firmware Interface
The Unified Extensible Firmware Interface is a specification that defines a software interface between an operating system and platform firmware...
.)
However it was quite a while before applications were updated to remove all 24-bit dependencies, and System 7 provided a way to switch back to 24-bit mode if application incompatibilities were found. By the time of migration to the PowerPC and System 7.1.2, 32-bit cleanliness was mandatory for creating native applications.
Object orientation
The rise of object-orientedObject-oriented programming
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
languages for programming the Mac – first Object Pascal
Object Pascal
Object Pascal refers to a branch of object-oriented derivatives of Pascal, mostly known as the primary programming language of Embarcadero Delphi.-Early history at Apple:...
, then later C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
– also caused problems for the memory model adopted. At first, it would seem natural that objects would be implemented via handles, to gain the advantage of being relocatable. However, these languages, as they were originally designed, used pointers for objects, which would lead to fragmentation issues. A solution, implemented by the THINK (later Symantec
Symantec
Symantec Corporation is the largest maker of security software for computers. The company is headquartered in Mountain View, California, and is a Fortune 500 company and a member of the S&P 500 stock market index.-History:...
) compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
s, was to use Handles internally for objects, but use a pointer syntax to access them. This seemed a good idea at first, but soon deep problems emerged, since programmers could not tell whether they were dealing with a relocatable or fixed block, and so had no way to know whether to take on the task of locking objects or not. Needless to say this led to huge numbers of bugs and problems with these early object implementations. Later compilers did not attempt to do this, but used real pointers, often implementing their own memory allocation schemes to workaround the Mac OS memory model.
While the Mac OS memory model, with all its inherent problems, remained this way right through to Mac OS 9
Mac OS 9
Mac OS 9 is the final major release of Apple's Mac OS before the launch of Mac OS X. Introduced on October 23, 1999, Apple positioned it as "The Best Internet Operating System Ever," highlighting Sherlock 2's Internet search capabilities, integration with Apple's free online services known as...
, due to severe application compatibility constraints, the increasing availability of cheap RAM meant that by and large most users could upgrade their way out of a corner. The memory wasn't used efficiently, but it was abundant enough that the issue never became critical. This is perhaps ironic given that the purpose of the original design was to maximise the use of very limited amounts of memory. Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
finally does away with the whole scheme, implementing a modern sparse virtual memory scheme. A subset of the older memory model APIs still exist for compatibility as part of Carbon
Carbon (API)
Carbon is one of Apple Inc.'s procedural application programming interfaces for the Macintosh operating system. It provides C programming language access to Macintosh system services...
, but map to the modern memory manager (a threadsafe malloc
Malloc
C dynamic memory allocation refers to performing dynamic memory allocation in the C via a group of functions in the C standard library, namely malloc, realloc, calloc and free....
implementation) underneath.
Apple recommends that Mac OS X code use malloc and free "almost exclusively".