Historically, the classic Mac OS used a form of memory management that has fallen out of favor in modern systems. Criticism of this approach was one of the key areas addressed by the change to Mac OS X.
Showing posts with label Mac OS X Snow Leopard. Showing posts with label Mac OS X Snow Leopard. Wednesday, November 9, 2016. MacBook 2,1 (2007): Mac OS X Snow Leopard: Installing rEFInd. Analytic number theory (1) Anamorphic format (1) Anatomy (1). My previous post dealt with how versioning and non-versioning apps interact under the new Mac OS X Lion release. I now turn to some low-level sleuthing. The motivation is, in a way, similar: I am interested in how command-line tools such as rsync may deal with versioning.
The original problem for the engineers of the Macintosh was how to make optimum use of the 128 KB of RAM with which the machine was equipped, on Motorola 68000-based computer hardware that did not support virtual memory.[1] Since at that time the machine could only run one application program at a time, and there was no fixedsecondary storage, the engineers implemented a simple scheme which worked well with those particular constraints. That design choice did not scale well with the development of the machine, creating various difficulties for both programmers and users.
Fragmentation[edit]
The primary concern of the original engineers appears to have been fragmentation – that is, the repeated allocation and deallocation of memory through pointers leading 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.[2] 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 memory available for this scheme – the system heap (used for the OS), and the application heap.[3] 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.
The memory management system had weaknesses; the system heap was not protected from errant applications, as would have been possible if the system architecture had supported memory protection, and this was frequently the cause of system problems and crashes.[4] 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. This was a real problem for almost every system API that existed. Because of the transparency of system-owned data structures at the time, the APIs could do little to solve this. Thus the onus was on the programmer not to create such pointers, or at least manage them very carefully by dereferencing all handles after every such API call. Since many programmers were not generally familiar with this approach, early Mac programs suffered frequently from faults arising from this.[5]
Palm OS and 16-bit Windows use a similar scheme for memory management, but 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, but 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.[6] In Palm OS and Windows, handles are an 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 keep 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[edit]
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 (which usually resulted in a hard crash—annoying on a single-tasking system, potentially disastrous if other programs are running).
Switcher[edit]
The situation worsened with the advent of Switcher, which was a way for a Mac with 512KB or more of memory to run multiple applications at once.[7] This was a necessary step forward for users, who found the one-app-at-a-time approach very limiting. 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.[8]The amount of actual RAM allocated to each heap was set by a value coded into the metadata of each application, set by the programmer. Sometimes 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. While popular among 'power users', this exposure of a technical implementation detail was against the grain of the Mac user philosophy. Apart from exposing users to esoteric technicalities, it was inefficient, since an application would be made to grab all of its allotted RAM, even if it left most of it subsequently unused. Another application might be memory starved, but would be unable to utilize the free memory 'owned' by another application.[3]
While an application could not beneficially utilize a sister application's heap, it could certainly destroy it, typically by inadvertently writing to a nonsense address. An application accidentally treating a fragment of text or image, or an unassigned location as a pointer 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 evolved into MultiFinder in System 4.2, which became the Process Manager in System 7, and by then the scheme was long 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 problems. Apple's System 7 Tune-up addon added a 'minimum' memory size and a 'preferred' size—if the preferred amount of memory was not available, the program could launch in the minimum space, possibly with reduced functionality. This was incorporated into the standard OS starting with System 7.1, but still did not address the root problem.[9]
Virtual memory schemes, which made more memory available by paging unused portions of memory to disk, were made available by third-party utilities like Connectix Virtual, and then by Apple in System 7. This increased Macintosh memory capacity at a performance cost, but did not add protected memory or prevent the memory manager's heap compaction that would invalidate some pointers.
32-bit clean[edit]
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 Plus. 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. 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.[10] 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.[4]
While a good use of very limited RAM space, this design caused problems when Apple introduced the Macintosh II, which used the 32-bit Motorola 68020 CPU. The 68020 had 32 physical address lines which could address up to 4 GB (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 IIci and later machines, HLock() and other APIs were rewritten to implement handle locking in a way other than flagging the high bits of handles. But many Macintosh application programmers and a great deal of the Macintosh system software code itself 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 machine 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 128 MB 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 drivers are loaded and executed. Older ROMs did not have any 32-bit Memory Manager 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.
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.[3] By the time of migration to the PowerPC and System 7.1.2, 32-bit cleanliness was mandatory for creating native applications and even later Motorola 68040 based Macs could not support 24-bit mode.[6][11]
Object orientation[edit]
The rise of object-oriented 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. 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) compilers, 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 work around 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 was not used efficiently, but it was abundant enough that the issue never became critical. This is ironic given that the purpose of the original design was to maximise the use of very limited amounts of memory. Mac OS X finally did away with the whole scheme, implementing a modern sparse virtual memory scheme. A subset of the older memory model APIs still exists for compatibility as part of Carbon, but maps to the modern memory manager (a thread-safe malloc implementation) underneath.[6] Apple recommends that Mac OS X code use malloc and free 'almost exclusively'.[12]
32-bit clean[edit]
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 Plus. 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. 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.[10] 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.[4]
While a good use of very limited RAM space, this design caused problems when Apple introduced the Macintosh II, which used the 32-bit Motorola 68020 CPU. The 68020 had 32 physical address lines which could address up to 4 GB (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 IIci and later machines, HLock() and other APIs were rewritten to implement handle locking in a way other than flagging the high bits of handles. But many Macintosh application programmers and a great deal of the Macintosh system software code itself 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 machine 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 128 MB 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 drivers are loaded and executed. Older ROMs did not have any 32-bit Memory Manager 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.
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.[3] By the time of migration to the PowerPC and System 7.1.2, 32-bit cleanliness was mandatory for creating native applications and even later Motorola 68040 based Macs could not support 24-bit mode.[6][11]
Object orientation[edit]
The rise of object-oriented 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. 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) compilers, 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 work around 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 was not used efficiently, but it was abundant enough that the issue never became critical. This is ironic given that the purpose of the original design was to maximise the use of very limited amounts of memory. Mac OS X finally did away with the whole scheme, implementing a modern sparse virtual memory scheme. A subset of the older memory model APIs still exists for compatibility as part of Carbon, but maps to the modern memory manager (a thread-safe malloc implementation) underneath.[6] Apple recommends that Mac OS X code use malloc and free 'almost exclusively'.[12]
References[edit]
- ^Hertzfeld, Andy (September 1983), The Original Macintosh: We're Not Hackers!, retrieved May 10, 2010CS1 maint: discouraged parameter (link)
- ^Hertzfeld, Andy (January 1982), The Original Macintosh: Hungarian, archived from the original on June 19, 2010, retrieved May 10, 2010CS1 maint: discouraged parameter (link)
- ^ abcmemorymanagement.org (December 15, 2000), Memory management in Mac OS, archived from the original on May 16, 2010, retrieved May 10, 2010CS1 maint: discouraged parameter (link)
- ^ abHertzfeld, Andy, The Original Macintosh: Mea Culpa, retrieved May 10, 2010CS1 maint: discouraged parameter (link)
- ^Apple Computer (October 1, 1985), Technical Note OV09: Debugging With PurgeMem and CompactMem, retrieved May 10, 2010CS1 maint: discouraged parameter (link)
- ^ abcLegacy Memory Manager Reference, Apple Inc., June 27, 2007, retrieved May 10, 2010CS1 maint: discouraged parameter (link)
- ^Hertzfeld, Andy (October 1984), The Original Macintosh: Switcher, retrieved May 10, 2010CS1 maint: discouraged parameter (link)
- ^Mindfire Solutions (March 6, 2002), Memory Management in Mac OS(PDF), p. 2, retrieved May 10, 2010CS1 maint: discouraged parameter (link)
- ^'System 7.1 upgrade guide'(PDF). Archived from the original(PDF) on March 4, 2016. Retrieved May 26, 2015.
- ^'memory maps'. Osdata.com. March 28, 2001. Retrieved May 11, 2010.CS1 maint: discouraged parameter (link)
- ^Apple Computer (January 1, 1991), Technical Note ME13: Memory Manager Compatibility, retrieved May 10, 2010CS1 maint: discouraged parameter (link)
- ^Memory Allocation Recommendations on OS X, Apple Inc, July 12, 2005, retrieved September 22, 2009CS1 maint: discouraged parameter (link)
External links[edit]
- Macintosh: ROM Size for Various Models, Apple Inc, August 23, 2000, retrieved September 22, 2009CS1 maint: discouraged parameter (link)
In my post of June 22, 2010 entitled Farmer Freud and Farmer Bowlby—Ploughing the Same Field? I ended with this quote by Bowlby (from vol. I, page xxvii): 'From a new viewpoint a familiar landscape can sometimes look very different.' The familiar landscape Bowlby is referring to here is none other than Freud's psychodynamic theory and thinking. On page xxxi, Bowlby tells us that 'Freud followed many different lines of thought and tried many possible theoretical constructions.' For an excellent treatment of how Freud tried to use the hydraulic model—very much in vogue back at the turn of the last century—to frame his work, see if you can't grab a copy of Professor Solomon's tape series Passions: Philosophy and the Intelligence of Emotions (available through the Teaching Company). Lecture eleven is entitled Freud's Catharsis—the Hydraulic Model. A bit further along in vol. I (page 3), Bowlby writes, 'During nearly fifty years of psychoanalytic investigations Freud tried first one then another point of view from which to start his inquiries.' As I mentioned in my June 22 post, recognizing that Bowlby and Freud looked at similar core issues (i.e., love and hate, anxiety and defence, attachment and loss) but framed them using worldviews diametrically opposed to each other, is rather important.
On page xxvii, Bowlby gives us the following quote from his famous WHO report Maternal Care and Mental Health:
What is believed to be essential for mental health is that the infant and young child should experience a warm, intimate and continuous relationship with his mother (or permanent mother-substitute) in which both find satisfaction and enjoyment.
Theory Mac Os Catalina
Critics of Bowlby's WHO report generally agreed that Bowlby (along with a number of colleagues) successfully demonstrated a correlation between ill effects in children and maternal deprivation. However, '[The WHO report] said very little indeed about the process whereby ill effects are brought into being' (quoting Bowlby). Early on Bowlby was faced with a dilemma: how to properly frame 'the process whereby ill effects are brought into being' so that he could gain a certain level of scientific respectability (which is what critics called for and Freud was not able to satisfactorily deliver). Today, to gain scientific respectability you are essentially forced to use a reductionistic model, that is to say, a model that calls for reducing all elements under investigation to simple billiard ball style 'cause and effect' chains. But the decision back in Bowlby's day wasn't as clear-cut as it is today. In the same way that the Mac operating system today challenges the Windows operating system by holding (roughly) 10 percent of the operating system market, back in Bowlby's day there was (metaphorically speaking) a Mac-esque challenge to the dominant reductionistic model. It was (and still is) called systems theory. Rather than reducing elements to causal chains, systems theory is about looking at elements in dynamic relationship with other elements in their natural environment (as opposed to a reduced lab environment).
Which 'operating system' (metaphorically speaking) to choose? This was Bowlby's dilemma. It's the same dilemma facing a computer buyer who goes into a big box store and sees rows and rows of PCs running Windows and one or two Macs off in the corner running the Mac OS. Even though the odds are stacked against them (in terms of widespread acceptance and convention) there's a hardy (some would say fanatic) group of Mac followers (myself included). Equally, back in Bowlby's day, there was a hardy (maybe even fanatic) group of followers who believed in the systems approach to science. The names may surprise you: Erik Erikson, Jean Piaget, Margaret Mead, Ludwig von Bertalanffy, Konrad Lorenz, Julian Huxley, Bäerbel Inhelder, Grey Walter, and the list goes on. For a behind the scenes look at this systems theory 'user group,' see the 1969 book Discussions on Child Development (edited by Tanner & Inhelder). This book is a collection of proceedings of a study group (which met from 1953 – 1956) where a fair amount of the focus was on systems theory, processes, and thinking. I'm pulling my observations from an unpublished paper (commissioned by our Foundation) by Dr. Gary Metcalf designed, in essence, to see how much of a systems theory fanatic Bowlby really was. Dr. Metcalf's paper has a working title of John Bowlby: Rediscovering a Systems Scientist. Turns out that Bowlby very much was a part of a systems theory movement, along with the luminaries mentioned above. Like picking a Mac today can put a computer user at a disadvantage (in terms of widespread acceptance and convention), so too picking the systems theory 'operating system' back in Bowlby's day. We get a sense for this risk when on page 41 of vol. I we hear Bowlby lament:
At one time to attribute purposiveness to animals or to build a psychology of human behavior on the concept of purposefulness [a key focus of systems thinking] was to declare oneself a vitalist [which is how Bowlby viewed Freud] and to be banned from the company of respectable scientists [who would no doubt use Windows if it were available to them].
Mac Os Catalina
I mention all of this because the systems road, the systems 'operating system' if you will, that Bowlby apparently used (at least according to Dr. Metcalf's research) is a road that isn't often talked about these days. As we continue our journey, I'll try to point out the artifacts in Bowlby's trilogy that mark this systems road in much the same way old gas stations and motels still mark where the old Route 66 used to pass.
