Turbo Pascal Memory
Written for beginners
New: Delphi pointer considerations
upd. Dec-25-1998
The article explains the properties of memory in DOS real mode Turbo Pascal programs. It does not explain programming very extensively, but it gives some background information to programmers how to use variables, typed constants and pointers.

Program / Data

The program is the list of instructions that tells the computer, what to do with the data. The data can be constants but most often the data are varying. That means that the program does the same procedures over and over with varying numbers (values) and strings.

On this early point it must be mentioned that the program is some kind of data too, but this is valid only for the operating system, when it loads the program from disk to RAM. From the standpoint of your application, the program is not data, it is the list of commands (instructions) which is absolutely static as long as the application is running in the computer.

Data can be stored in fast RAM and on the disk. The disk space is virtually unlimited, but it is somewhat more complicated to maintain, compared with the data in RAM.

Data in RAM are held in "VARIABLES", named locations whose addresses are managed by the program or, more precisely, by the compiler. Indeed the names of the variables are known to the compiler only, they are not saved in the .EXE program file. In the running machine program the variables simply have addresses.

Kinds of data

Constant data are saved in a code segment, i.e. the memory range where the program itself is located. The code segments are explained below. Note that "typed constants" are not constants but variables, see below.

Variable data are divided in several kinds.

Global data are the variables which the programmer declared on top of the program or unit with the Pascal "Var" instruction. They reside in the DS: data segment, which is limited to max. 64 kB. They occupy memory space as long as the application program runs.
There are typed constants in Turbo Pascal, made with the Const instruction instead of the Var instruction. These are by definition normal variables in the data segment, they simply are "filled" with a startup value. Example:
  Const COMport : Integer = 1;
Typed constants are a Turbo Pascal specialty!

Local data are the variables which the programmer declared in a procedure or function below the Procedure declaration and before the Begin -- End; clause. They are alive only as long as the procedure or funcion is running in its begin - end; clause. On entry at Begin the value in the variables is undefined. The total available memory space is limited to the value which was declared with the $M(xxxx, ....) compiler instruction (usually 16kB or 32kB), and the programmer must be aware that all procedures which call each other can add more variables. On the other hand most local variables do not "live" very long since procedures return early and thus release the obtained stack memory. The stack not only holds the local variables but also the return addresses of the procedures, needing  6 bytes per call, but this is usually not a high value except in recursive procedures. The parameters to the procedures and the function results also occupy some space on the stack.
The formal procedure / function parameters are very similar to local variables in the called (=running) procedure.

Heap data are mass data which are held by a pointer, the memory was obtained with a New(ppp) or GetMem(ppp,sss) instruction. They are alive until the pointer is given back to the pool with Dispose(ppp) or FreeMem(ppp, sss). Usually the pointers reside in the global data area, the DS: segment, but there they occupy 4 bytes each only.

PC Memory layout

The available memory for the application program is limited on the low end and on the high end by particular properties which were fixed when the PC was developed some 15 years ago. The PC has 1 MB of memory available in Real mode and in the virtual real mode when running in a Windows DOS-box etc.

The low 1 kB is used for the interrupt vectors, this was defined by Intel for the 8086/8088 processor.

The ROM-BIOS data area is just above, beginning at segment address $0040. It contains the keyboard typeahead buffer and the cursor position on the screen and similar important data. One of the possibly interesting locations is the Byte $0040:$0017 where the shift+caps+ctrl+alt+numlk bits are located and at $0040:$006C is a longint which increments automatically with 18.2 Hz, it can be used for timing and processor independent delay applications. At midnight it is reset to 0 automatically.

The ROM-BIOS (shorthand ROS) is the program which is part of the PC from the manufacturer, it contains the necessary programs to handle the keyboard, the disks and the CRT in textmode for the boot process. The machine code of the ROM BIOS is placed on top of the 1MB memory area and it can be expanded downwards by several extensions, eg. the SCSI driver routines, the silly plug&play and the BIOS password code or by network card drivers or similar code which must be present at boot time.

Above the ROS data, which usually only is some 512 bytes, the area for the IO.SYS and the drivers from CONFIG.SYS is located. Modern drivers can be moved into the high memory area above C800 to keep the space requirements in the most important low memory as small as possible.
Depending on the amount and qualitiy of the .SYS drivers the bottom DOS-memory ends somewhere at $0A00 until $1200.

Above the DOS/SYS memory the application program is loaded and it can occupy memory up until $A000. This is the "famous" 640k limit. The total amount of memory available for the application program is typically 540kB up to 600 kB.

Above the application programs memory the video RAM is mapped into the 1MB memory area, followed by the video VGA-BIOS in ROM. This is a special extension to the ROM BIOS which was introduced when the very old CGA and MDA graphics were enhanced to EGA and VGA.

Above the VGA-BIOS is some memory which can be used by drivers (MOUSE.SYS, KEYB.COM etc), the "high memory". MSDOS is usually placed above 1 MB by a special hardware trick which is called the "A20" switch. It is not explained here.

Application memory layout

The first known area is the PSP memory area, it is 256 bytes and its layout is historically fixed first by the archaic Intellec system of the 8008 CPU and later by the CP/M operating system. The programs in these machines started at $0100 in absolute memory, which was 64kB total! The PSP area is principally the "descriptive" area of the running application program, and it can be accessed by skilled programmers for particular special purposes. Beginners shall not even think of manipulating any location in the PSP.

Above the PSP the loader of the operating system places the code of the program. It is the invariable part of the whole application program. It is referred to as the CS: code segment, but it can contain several distinct CS: segments, from the main program and for every unit and for the system unit of the run time library. Each single code segment can be up to 64kB but there is no limitation on how many CS: segments are used. The programmer need not take care of the CS: segments usually, it is sufficient to take care of the units in the program.

The following data segment DS: is used for all the global variables. They are named global because they are accessible by all procedures as opposed to the local variables which are "unknown" in another procedure. The data segment is limited to 64kB. It must be mentioned here that global variables can be declared in the implementation part of a unit. In this case they are local to all procedures and functions in the particular unit and "unknown" in other units. But they are treated as global variables since they are located in the DS: data segment - and their behaviour to all the procedures in the particular unit is a global one. Their memory address in the data segment is an add-on to the really global variables which are declared in the Interface part of the units. These are a Borland Turbo Pascal specialty and are not fixed in the Pascal standards. Another TP specialty are the typed constants in the data segment, as mentioned above. The typed constants are stored within the .EXE program file and loaded to RAM at program start at the bottom of the DS: data segment. The other variables are simply placed above and cleared to 00 at program start (not proven for all versions of TP!).

If you use dynamic objects additional data area is occupied for each type of objects.

The stack segment SS: holds the local data and the return addresses and the parameters of the procedures. Its size cannot be calculated by the compiler, because it does not "know" at compile time which procedures can call each other and put their local variables on the stack. It must be estimated by the programmer and it can be checked in test runs of the application program with the $S switch ON. The stack pointer is initially set to the top of the stack memory, eg. to $4000 for 16kB stack, growing "downwards" whenever procedures are called. The SPtr function can tell you how far it has grown down towards 0000. There shall be a minimum of 2 kB remaining ($800) for possible hardware interrupts etc at any time in the program, but this is somewhat a philosophical value, other people say that 256 bytes are sufficient.

The heap is the remaining memory above the stack. Its minimum size must be estimated by the programmer and its maximum size can be defined with the $M switch too. At run time the remaining heap space can be checked with the MemAvail function from the system - tpu. The heap data area can be managed in the program with pointers, where New(ppp) or GetMem(ppp,sss) are the run time procedures which obtain a memory area from the heap manager. The result of the New(ppp) procedure is an address filled into the pointer variable - or a program crash if there is not enough free memory available on the heap. If you need large blocks of memory you better ask the MaxAvail function instead of MemAvail before you invoke New or GetMem.

It is not wise to occupy heap space in tiny slizes, eg. for single integer numbers. Pointers are best used with arrays and records of data. The New(ppp) procedure occupies memory in 8-byte slizes, so it is wise to declare strings as multiples of 8 bytes, e.g. String[7] or String[15] or String[31]. Together with the length byte they occupy 8, 16 or 32 bytes. The 8-byte increment is used by the heap manager for its internal garbage collection when disposed memory blocks are linked together.

Working with the heap is in some way similar to working with data in files. The programmer must know which pointers are valid in the moment, i.e. have real memory available. Note also that the Dispose(ppp) and FreeMem(ppp,sss) procedures do not set the pointers to Nil.

Programs which use the overlay technique place some code in the heap. The overlay area is occupied at program start and simply reduces the available heap space for data. On the other hand it helps to reduce the amount of necessary CS: memory at large programs and thus increases the overall memory space on the heap.

If you want to call other application programs with the Exec procedure you must reduce the maximum heap space with the $M compiler option or reduce the heap dynamically, which is a rather sophisticated procedure!


Pointer primer


This is not a complete tutor about pointers, it shall help simply over the first obstacles. It is strongly suggested that novice programmers obtain any books and FAQs available, not only about pointers.

Pointers in Pascal are (mainly) the tool to
1) obtain memory from the heap
2) give access to the memory in the program

Pointers are a special kind of variables. They do not hold the data (integer ... Record ... Array ... Object) but simply the address of the data in memory. Pointers consist internally of 2 words, the segment and the offset of the address. This has to do with the way how the 8086 etc. processors manage the 1 MB memory area in Real Mode.

For the very first consideration you can treat a pointer as an ARRAY INDEX into the computer's memory. Assume the memory were a linear array [0..1M] of Byte similar to
Type   Memory = Array[0..1000000] of byte;  (this is extremely inaccurate, I know)
Var MemPointer : Longint;

  MemPointer := $00321F2;
  SomeByteFromMemory := Memory[MemPointer];

Using a longint to index into the memory is not possible with the real-mode 8086 processors, because it treats memory not in a flat mode, but in segments, but this is not so important for the very first understanding.

With the primitive example above you see how the MemPointer variable holds the pointer into a particular byte in memory. Pointer usage in actual programming is mainly the task how to get a proper value to store in the pointer variable, and how to use it properly. But this is true in any case: the pointer indexes into the memory area.

Now back to Pascal programming

The Pascal language conventions offer a very comfortable way to declare and use pointers, even for complex data structures, not only for simply byte arrays as shown above. The main benefit is the Type declaration of Pascal.

Assume a simple Record:
Type TSimpRec = Record
                  Name : String;
                  Age  : Integer;
Now you can declare variables of the type:
  Var Pers1, Pers2 : TSimpRec;
this will occupy memory in the data segment as described above, for 2 records.

  Var PPers1, PPers2 : ^TSimpRec;
will make two pointers, where each uses 4 bytes of data memory only.  On this point in the program the location of the 8 bytes in the program's data area is allocated, but not filled with a proper value. The pointers are invalid here.

Now you can put the address of the records into the pointers:
  PPers1 := Addr(Pers1);
  PPers2 := Addr(Pers2); (or shorthand: @Pers2)
This code piece can be used even if the contents of the PersX records did not contain any useful data.

Now you can access the records in 2 ways:
  Pers1.Name := 'Luciano Pavarotti';
  Pers1.Age  := 43; {or higher?}
but you can also access the record using the pointer, which was previously set to the address of Pers1:
  Writeln(PPers1^.Name,'  Age:', PPers1^.Age);

or even more elegant (this is not a pointer property, but a property of the record which the pointer inherited):
  with PPers1^ do
    Writeln(Name,'  Age:',Age);

You must notice that in Pascal a pointer has the same stringent type checking properties as the variable type to which it points.

The Pascal language conventions allow to define not only the usual types, like Integer, Char, Record etc. but also declare Pointer - types. The example above could be rewritten:
Type TSimpRec = Record
                  Name : String;
                  Age  : Integer;
     PSimpRec = ^TSimpRec;
Var Pers1,Pers2   : TSimpRec;
    PPers1,PPers2 : PSimpRec;   {instead of  ^TSimpRec}
This means that PSimpRec is derived from TSimpRec.

And, very interesting, the line PSimpRec = ^TSimpRec can be written ahead of the TSimpRec declaration.
Type PNameRec = ^TNameRec;  {defined ahead of the record definition}
     TNameRec = Record
                  Name : String;
                  Age  : Integer;
                  Next : PNameRec {so it can be used inside the record definition}

{For the "untyped" Pointer (used very seldom for special purposes only) and Nil look in your manual please.}

Heap memory

The example above (with the Addr() instruction) is not used in Pascal very frequently. Pointers are mainly used to obtain memory from the heap, which is a big pool of memory.

The heap manager is built in the run time library (SYSTEM.TPU) of Pascal. There are 4 built in procedures:
    Procedure New(Var P : Pointer);   (can be used as function too, especially for objects with an Init procedure, not explained here)
    Procedure Dispose(P : Pointer);    (can include a Done procedure for objects, not explained here)
    Procedure GetMem(Var P : Pointer;size:Word);
    Procedure Freemem(P : Pointer;size:Word);
and 2 functions
    function MemAvail : Longint;  and  function MaxAvail : Longint;
In fact the New and Dispose procedures are special, the compiler "knows" the size of the variable where the new pointer shall point to, by the type properties described above. They cannot be made by the application programmer. This is similar to the Write procedure, which cannot be made by the programmer too.

New and GetMem can be used to "create" variables dynamically, they are alive only as long as you need them. This is usually longer than the local variables, but seldom during the whole program life.

Pointers can be part of a record and it is also allowed to maintain an array of pointers. But the programmer is responsible for the validity of the pointers, that means that it is strongly forbidden to use a pointer with ^ before it got real memory with New!

Pointers which do not point to a living memory space shall be set to Nil for easy testing. Note that Dispose does NOT fill Nil into the pointer!
   PPers2 := Nil;   {invalidate the pointer}
.... testing:
   if PPers2 <> Nil then
     with PPers2^ do
It is not "necessary" that invalid pointers are filled with Nil, but it is usually good for programming.

The pointer is the only vehicle (handle) inside the program, that holds the address of the memory which was obtained from the heap, so use it with care. It is not very usual to assign values directly to pointers with :=  but it can be done, if you know what you are doing. But do not overwrite a pointer which holds an address from the heap, else the memory is lost, it can never be released with Dispose again. This is one of the reasons why typical Windows application programs cannot run for days, the programmers forgot to dispose the memory, so it is filled up with garbage until all memory is occupied. In Windows the heap memory is obtained from the operating system, not from a private pool of the application program.

It is important to consider that pointers contain valid data only between the New(ppp) procedure (and filling the memory with data) and Dispose. After dispose the pointer still exists (as a variable), but it is invalid and must not be used until it gets new memory which it points to with another New(ppp).

Program flow:
   New(ppp);    {obtain memory from the heap}
     ppp^ := something;        {use the memory, the pointer is the handle ...}
                        {here you can use the memory }
     another := ppp^;
     SomeProc(ppp^);    {also in procedures as var parameter...}
     AnotherProc(ppp);         {or simply as pointer}
   Dispose(ppp);        {"tilt" it}
From this point the data is no longer alive, the pointer is invalid! It is a very common source for system crashes to use invalid pointers!

Another very exciting property of Pointers is the fact that they can be the result of functions. It is (normally) not possible to have a record or an array as a function result (a string-function is a Borland TP exception/specialty!), but when the function has the New(ppp) built in, it can deliver a record or whatever, setup with the proper contents. (With Delphi any record can be a function result, but internally Delphi creates a pointer!)

Pointers are sometimes used by sophisticated programmers to get the address of procedures and functions in the program's code segment. One of the typical uses of Procedure Pointers is the ExitProc. If it is used, the procedure MUST be declared as FAR, to get the segment AND offset of the procedure. Another use of procedure pointers (in TP: procedure variables = variabls of "type procedure") is their use as parameters in other procedures. It means that you can have a procedure which does distinct things on data, yet this leads directly to objects.

Repetition from above:
With pointers the pascal compiler allows a particular exception from the rule, that a type cannot be declared forward. Indeed you can
Type PPerson = ^TPerson;
     TPerson = Record
                 Name : String;
                 Age  : Integer;
                 Next : PPerson;
The Next element is the main reason for this benefit. So you can create chained records. It is not really a forward declaration, since PPersion IS the pointer type of TPerson, not a derived type, by definition (so much philosophy?).

HINT: do not try to hold all your data in RAM (eg. on the heap) unless it is really necessary. Usually it is much better and easier to hold the data in a file, except if you really need frequent access to the data, eg. for searching and sorting. It is very usual to hold only some kind of directory or index in RAM, while the main records are on disk. It is very easy to have an array of Longints containing record numbers for the  Seek(F,...) procedure instead of an array of pointers to records in the expensive heap memory.

Using pointers with the Inline Assembler:
The CPU machine code handles pointers as Segment:Offset with registers.
Type TMyRec = Record
               Name : String;
               Age  : Integer;

Var BigVar : Pointer;
    MyPtr  : ^TMyRec;

  LES   DI,BigVar   {loads ES and DI simultaneously}
  MOV   BX,2300     {byte index into untyped BigVar}
  MOV   AX,ES:[BX+DI] {word 0..1150..4999, nota bene!}
  LES   SI,MyPtr    {can be BX, SI, DI.
                     Caution: never use LDS, except
                     you know what you are doing!!!
                     ES is the scratchpad segment register}
  MOV   AX,ES:[SI].TMyRec.Age   {index into typed MyPtr}

The following explanation is somewhat odd for beginners, but I saw that it can highlight the context.

You should know that Var - parameters to procedures are very similar to pointers. The compiler sets the address of the actual parameter into the parameter list which is posted to the called procedure. But you must not use the parameter with ^ as usual with parameters, the compiler makes the necessary indirection. If Pascal did not have the Var parameters they could easily be faked with the pointer method.

Procedure Add5(Var I : Integer);
   I := I+5;
End;  this is the normal way.

Var K : Integer;

using the pointer method:
Procedure Add5(P : ^Integer);
   P^  :=  P^ + 5;

Invoke it with
   Add5(Addr(K));   or   Add5(@K); {Borland shorthand notation for Addr(xxx)}

Var parameters give the procedure "write" access to the data. In functions this modification of parameters is often named a "side" effect, since the main job of the function is to get the function result. On the other hand, functions are often used instead of procedures with a simple boolean result, representing the "ok" of the operation, eg. read a var record from a file. In this case the function result is more "side effect", while the filled var record is what the programmer wanted.

  ANOTHER POINTER TUTORIAL for those of you who did not understand my kind of english...

Delphi and pointers

These paragraphs are written for Turbo Pascal programmers, who intend to write application programs for Windows, using the Windows version of Turbo Pascal named Delphi.

The most important difference between the Turbo Pascal heap management and the Delphi heap management comes from the Windows operating system. While with the DOS - TP the heap is completely managed by the RealTimeCode of the TP-application program without any notification of the operating system, a Windows application program obtains the heap blockwise from Windows on demand. This is managed automatically by the heap manager and so it is  usually invisible to the programmer. But it is important to consider the tight interconnection between the program and Windows, very similar to file operations etc.

It is very important that Windows programs dispose the memory after use to give it back to the pool for companion application programs, running concurrently in the multitasking / networking system. It is very bad programming style to create all forms (windows, dialogs) on program start, only displaying them on demand, occupying memory which could be used by other concurrently running application programs.

Delphi programs make extensive use of pointers and the heap memory, but unfortunately this is hidden in most cases, invisible to the programmer. There are many Delphi programmers (typically upgraders from Visual Basic) who do not even know that they are using pointers extensively in their programs.

Type TMyForm = class(TForm)
                 .... variables
                 .... methods

Var MyForm : TMyForm;

Indeed   MyForm is a pointer!
but this is invisible to the programmer. It is a small variable in the data memory area, pointing to the living instance of the form's memory on the heap (in the Windows memory area).

In pure Turbo Pascal this would be written as:

Type PMyForm = ^TMyForm;
     TMyForm = Object(TForm)    {a Delphi class is very similar to a TP object}
                 .... variables
                 .... methods

Var MyForm : PMyForm;

Delphi not only hides the pointer property (caution, property is used with a very distinct meaning in Delphi, but here it is used in a literal manner) of the MyForm pointer variable, but also performs the New() procedure in some hidden code when the Create - constructor is invoked.

  MyForm := TMyForm.Create(Application);
actually is translated to executable machine code as:
  MyForm.Create(Application);  {the Application parameter is not important in this context)

or (as you might be familiar with TP-object - constructors as usual eg. with Turbo Vision):
  MyForm := New(PMyForm,Create);

The  .Free - destructor performs the Dispose operation inherently in a very similar manner.

The Delphi compiler need not see the ^ symbol in your source code to de-reference the pointer, it dereferences it automatically when you are using a variable that is known to the compiler to be a pointer. This is very odd for calibrated/educated Pascal programmers!

Note: This couple of paragraphs refers to Objects/Classes as if they were commonly known. Indeed there is some "interference" between the pointer issues and the Objects, but it is not directly related. But this article deals with memory and pointers, not with Objects, which are simply treated as "well known".

More exotic memory:

EMS memory  (LIM expanded memory)
is a somewhat outperformed method of older PC generations to get additional memory for variables. The key problem of the older 8086 CPU was the limitation to access only 1MB of memory with its addressing methods. This limitation also inherited the 16-bit code of real mode programs. The trick of EMS is a "window" in the high memory area (eg. above the C800 VGA-BIOS) with 64kB in size. A particular hardware on the EMS memory card could map in memory to this address area in 16kB pages. By switching in and out various pages of a 1MB or 4MB special memory card the programmer could make use of this large area, but it needs some procedures to maintain the page mapping similar to the seek() instruction for disk files. The switching was supported by a special EMMxxx.SYS driver that has an interrupt vector entry with several distinct functions. The benefit was higher speed compared with disk files, the drawback was the necessary overhead and management procedures in the application program.
Modern computers are sometimes equipped with EMS properties without special hardware. The EMM386.SYS driver uses the page mapping capabilities of the 386 and later CPUs. But anyway, the 64k area at C800 or D000 or whatever is used as a "window" of the large extended memory into the 1MB area.
Turbo Pascal can use the EMS memory in two ways: Either as buffer for overlay management, where the whole overlay file is copied to EMS for speed and by the EMS stream in the Turbo Vision environment. Other usage of EMS must be programmed from scratch.

XMS memory  (extended memory)
is another outperformed method of accessing memory above 1 MB, it was introduced with the PC-AT and the 80286 CPU. XMS is seldom used with Turbo Pascal programming.

Modern computers and operating systems have another method to access the larger 4MB or more RAM, the DPMI approach. It is available on newer DOS versions with a special driver and in Windows 3.x - DOS-Boxes. It means that the program is running in an emulated protected mode operating system. This can be used with Borland Pascal 7 (not Turbo Pascal 7). The benefit of DPMI is the simplicity of using the New(ppp) as usual to get any amount of memory without having to deal with drivers and mappers etc.
With DPMI the segment part of the pointer is no longer the linear address / 16 paragraph number as described below, but an index into a system table (GDT or LDT) maintained by the 80386++ aware operating system. But it is also a "16-bit" segment with a size of max. 65536 bytes.

Segment and Offset - and Paragraphs

In REAL mode, the mode which was introduced in the early 80ies with the Intel 8086 and 8088 processors, the 1 MB memory is addressed with segments. Because with a 16 - bit address only 65536 bytes can be addressed (which was the total memory space of the 8080 and Z80 processors) a method had to be introduced to address 1024 kB. Intel decided to use the segmentation method.

A segment is a memory space of 64kB of memory, so it would have been sufficient to have 16 segments to cover the 1 MB memory area. But the Intel engineers were clever, they used a 16-bit segment value, such that 64k distinct segments could be defined. The segment points to a 16-byte paragraph, thus 64k overlapping segments were established. A paragraph is a memory area with 16-bytes, but this is a nomenclature consideration only.

A particular linear address can be pointed to with many different seg:ofs word values, eg.
$0400:$0200 points to the same address as $0420:$0000 and $0410:$0100 ==>   $04200 as linear address.

The seg:ofs writing is standardized with the Intel familiy of processors in 8086 mode.

There is no need in a typical application program to know the absolute, linear address and in most cases the programmer need not be aware of the contents of the used pointers as numerical values. In the graphic picture above the segment values are used to show the address.

Franz Glaser, Austria
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TP-FAQ      Dynamic arrays
Sorry, my English is not perfect, but I hope that you will understand the explanations anyway.


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