CHAPTER 6 Programming Technical Reference - IBM Copyright 1988, Dave Williams DOS CONTROL BLOCKS AND WORK AREAS When DOS loads a program, it first sets aside a section of memory for the program called the program segment, or code segment. Then it constructs a control block called the program segment prefix, or PSP, in the first 256 (100h) bytes. Usually, the program is loaded directly after the PSP at 100h. The PSP contains various information used by DOS to help run the program. The PSP is always located at offset 0 within the code segment. When a program recieves control certain registers are set to point to the PSP. For a COM file, all registers are set to point to the beginning of the PSP and the program begins at 100h. For the more complex EXE file structures, only DS and ES registers are set to point to the PSP. The linker passes the settings for the DS, IP, SS, and SP registers and may set the starting location in CS:IP to a location other than 100h. IBMBIO provides an IRET instruction at absolute address 847h for use as a dummy routine for interrupts that are not used by DOS. This lets the interrupts do nothing until their vectors are rerouted to ttheir appropriate handlers. A storage block is used by DOS to record the amount and location of allocated memory withion the machine's address spacd. A storage block, a Program Segment Prefix, and an environment area are built by FDOS for each program currently resident in the address space. The storage block is used by DOS to record the address range of memory allocated to a program. IOt us used by DOs to find th enext availible area to load a program and to determine if there is a\enough memory to run that porogram. When a memory area is in use, it is said to be allocated. Then the program ends, or releases memory, it is said to bne deallocated. A storage block contains a pointer ro rhe Program Segment Prefix assoiciated with each program. This control block is constructed by IBMDOS for the purpose opf providing stanfdardized areas for DOS/program communication., Within ghr PSP are arsas which are used to save interrupt vectors, pass parameters to the program, record disk directory information, and to buffer disk reads and writes. This control block is 100h bytes in lengrth and is followed by the program mopdule loaded by DOS. The PSP contains a pointer to the environment area for that program. This area contains a copy of the current DOS SET, PROMPT, COMSPEC, and PATH values as well as any user-set variables. The program may examine and modify this information as desired. Each storage block is 10h bytes long, although only 5 bytes are currently used by DOS. The first byte contains 4Dh (a capital M) to indicate that it contains a pointer to the next storage block. A 5Ah (a capital Z) in the first byte of a storage block indicatres there are no more storage blocks following this one (it is the end of the chain). The identifier byte is followeed by a 2 byte segment number for the associated PSP for that program. The next 2 bytes contain the number of segments what are allocated to the program. If this is not the last storage block, then another storage block follows the allocated memory area. When thge storage block contains zero for the nuymber of allocated segments, then no storage is allocated to thius block and the next storage block immediately follows this one. This can ha-p[en whjen memory is allocated and then deallocated repeatedly. IBMDOS constructs a storage block and PSP before loading the command interpreter (default is COMMAND.COM). If the copy of COMMAND.COM is a secondary copy, it will lack an environment address as PSP+2Ch. The Disk Transfer Area (DTA) DOS uses an area in memory to contain the data for all file reads and writes that are performed with FCB function calls. This are is known as the disk transfer area. This disk transfer area (DTA) is sometimes called a buffer. It can be located anywhere in the data area of your application program and should be set by your program. Only one DTA can be in effect at a time, so your program must tell DOS what memory location to use before using any disk read or write functions. Use function call 1Ah (Set Disk Transfer Address) to set the disk transfer address. Use function call 2Fh (Get Disk Transfer Address) to get the disk transfer address. Once set, DOS continues to use that area for all disk operations until another function call 1Ah is issued to define a new DTA. When a program is given control by COMMAND.COM, a default DTA large enough to hold 128 bytes is established at 80h into the program's Program Segment Prefix. For file reads and writes that are performed with the extended function calls, there is no need to set a DTA address. Instead, specify a buffer address when you issue the read or write call. DOS Program Segment When you enter an external command or call a program through the EXEC function call, DOS determines the lowest availible address space to use as the start of available memory for the program being started. This area is called the Program Segment. At offset 0 within the program segment, DOS builds the Program Segment Prefix control block. EXEC loads the program after the Program Segment Prefix (at offset 100h) and gives it control. The program returns from EXEC by a jump to offset 0 in the Program Segment Prefix, by issuing an int 20h, or by issuing an int 21h with register AH=00h or 4Ch, or by calling location 50h in the PSP with AH=00h or 4Ch. It is the responsibility of all programs to ensure that the CS register contains the segment address of the Program Segment Prefix when terminating by any of these methods except call 4Ch. All of these methods result in returning to the program that issued the EXEC. During this returning process, interrupt vectors 22h, 23h, and 24h (Terminate, Ctrl-Break, and Critical Error Exit addresses) are restored from the values saved in the PSP of the terminating program. Control is then given to the terminate address. When a program receives control, the following conditions are in effect: For all programs: 1) The segment address of the passed environment is contained at offset 2Ch in the Program Segment Prefix. 2) The environment is a series of ASCII strings totalling less than 32k bytes in the form: NAME=parameter The default environment is 160 bytes. Each string is terminated by a byte of zeroes, and the entire set of strings is terminated by abother byte of zeroes. Following the byte of zeroes that terminates the set of environment string is a set of initial arguments passed to a program that contains a word count followed by an ASCIIZ string. The ASCIIZ string contains the drive, path, and filename.ext of the executable program. Programs may use this area to determine where the program was loaded from. The environment built by the command processor (and passed to all programs it invokes) contains a COMSPEC=string at a minimum (the parameter on COMSPEC is the path used by DOS to locate COMMAND.COM on disk). The last PATH and PROMPT commands issued will also be in the environment, along with any environment strings entered through the SET command. The environment that you are passed is actually a copy of the invoking process's environment. If your application terminates and stays resident through int 27h, you should be aware that the copy of the environment passed to you is static. That is, it will not change even if subsequent PATH, PROMPT, or SET commands are issued. The environment can be used to transfer information between processes or to store strings for later use by application programs. The environment is always located on a paragraph boundary. This is its format: byte ASCIIZ string 1 byte ASCIIZ string 2 .... byte ASCIIZ string n byte of zeros (0) Typically the environment strings have the form: parameter = value Following the byte of zeros in the environment, a WORD indicates the number of other strings following. If the environment is part of an EXECed command interpreter, it is followed by a copy of the DS:DX filename passed to the child process. A zero value causes the newly created process to inherit the parent's environment. 3) Offset 80h in the PSP contains code to invoke the DOS function dispatcher. Thus, by placing the desired function number in AH, a program can issue a long call to PSP+50h to invoke a DOS function rather than issuing an int 21h. 4) The disk transfer address (DTA) is set to 80h (default DTA in PSP). 5) File Control Blocks 5Ch and 6Ch are formatted from the first two parameters entered when the command was invoked. Note that if either parameter contained a path name, then the corresponding FCB will contain only a valid drive number. The filename field will not be valid. 6) An unformatted parameter area at 81h contains all the characters entered after the command name (including leading and imbedded delimiters), with 80h set to the number of characters. If the <, >, or | parameters were entered on the command line, they (and the filenames associated with them) will not appear in this area, because redirection of standard input and output is transparent to applications. (For EXE files only) 7) DS and ES registers are set to point to the PSP. 8) CS, IP, SS, and SP registers are set to the values passed by the linker. (For COM files only) 9) For COM files, offset 6 (one word) contains the number of bytes availible in the segment. 10) Register AX reflects the validity of drive specifiers entered with the first two parameters as follows: AL=0FFh is the first parameter contained an invalid drive specifier, otherwise AL=00h. AL=0FFh if the second parameter contained an invalid drive specifier, otherwise AL=00h. 11) All four segment registers contain the segment address of the inital allocation block, that starts within the PSP control block. All of user memory is allocated to the program. If the program needs to invoke another program through the EXEC function call (4Bh), it must first free some memory through the SETBLOCK function call to provide space for the program being invoked. 12) The Instruction Pointer (IP) is set to 100h. 13) The SP register is set to the end of the program's segment. The segment size at offset 6 is rounded down to the paragraph size. 14) A word of zeroes is placed on top of the stack. The PSP (with offsets in hexadecimal) is formatted as follows: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 � P R O G R A M S E G M E N T P R E F I X � 쳐컴컴컴쩡컴컴컴컴컫컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � offset� size � C O N T E N T S � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0000h � 2 bytes � int 20h � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0002h � 2 bytes � segment address, end of allocation block � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0004h � 1 byte � reserved, normally 0 � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0005h � 5 bytes � long call to MSDOS function dispatcher � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 000Ah � 4 bytes � previous termination handler interrupt vector (int 22h) � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 000Eh � 4 bytes � previous contents of ctrl-C interrupt vector (int 23h) � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0012h � 4 bytes � prev. critical error handler interrupt vector (int 24h) � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0016h � 22 bytes � reserved for DOS � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 002Ch � 2 bytes � segment address of environment block � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 002Eh � 34 bytes � reserved, DOS work area � 읕컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 4 bytes � stores the calling process's stack pointer when � � � switching to DOS's internal stack. � 旼컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0050h � 3 bytes � int 21h, RETF instructions � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0053h � 2 bytes � reserved � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0055h � 7 bytes � reserved, or FCB#1 extension � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 005Ch � � default File Control Block #1 � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 006Ch � � default File Control Block #2 (overlaid if FCB #1 opened) � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0080h � 1 byte � parameter length � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0081h � � parameters � 쳐컴컴컴탠컴컴컴컴컵컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 00FFh � 128 bytes� command tail and default Disk Transfer Area (DTA) � 읕컴컴컴좔컴컴컴컴컨컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸 1. The first segment of availible memory is in segment (paragraph) form. For example, 1000h would respresent 64k. 2. The word at offset 6 contains the number of bytes availible in the segment. 3. Offset 2Ch contains the segment address of the environment. 4. Programs must not alter any part of the PSP below offset 5Ch. offset 0 contains hex bytes CD 20, the int 20h opcode. A program can end by making a jump to this location when the CS points to the PSP. For normal cases, int 21, function 4Ch should be used. offset 2 contains the segment-paragraph address of the end of memory as reported by DOS. (which may not be the same as the real end of RAM). Multiply this number by 10h or 16 to get the amount of memory availible. offset 4 reserved offset 05 contains a long call to the DOS function dispatcher. Programs may jump to this address instead of calling int 21 if they wish. offsets 10, 14, 18 vectors offset 2C is the segment:offset address of the environment for the program using this particular PSP. offset 2E The DWORD at PSP+2EH is used by DOS to store the calling process's stack pointer when switching to DOS's own private stack - at the end of a DOS function call, SS:SP is restored from this address. offset 50h contains a long call to the DOS function dispatcher. offsets 5C, 65, 6C contain FCB information for use with FCB function calls. The first FCB may overlay the second if it is an extended call; your program should revector these areas to a safe place if you intend to use them. offset 80h and 81h contain th elength and value of parameters passed on the command line. offset FF contains the DTA STANDRD FILE CONTROL BLOCK The standard file control block is defined as follows, with the offsets in decimal: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 � F I L E C O N T R O L B L O C K � 쳐컴컴컴쩡컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � Bytes � Function � 쳐컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0 � Drive number. For example: � � � Before open: 00h = default drive � � � 01h = drive A: � � � 02h = drive B: etc. � � � After open: 00h = drive C: � � � 01h = drive A: � � � 02h = drive B: etc. � � � 0 is replaced by the actual drive number during open. � 쳐컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 1-8 � Filename, left justified with trailing blanks. If a reserved device � � � name is placed here (such as PRN) do not include the optional colon. � 쳐컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 9-11 � Filename extension, left justified with trailing blanks. � 쳐컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 12-13 � Current block number relative to beginning of file, starting with 0 � � � (set to 0 by the open function call). A block consists of 128 � � � records, each of the size specified in the logical record size field.� � � The current block number is used with the current record field � � � (below) for sequential reads and writes. � 쳐컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 14-15 � Logical record size in bytes. Set to 80h by the OPEN function call. � � � If this is not correct, you must set the value because DOS uses it � � � to determine the proper locations in the file for all disk reads and � � � writes. � 쳐컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 16-19 � File size in bytes. In this 2 word field, the first word is the � � � low-order part of the size. � 쳐컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 20-21 � Date the file was created or last updated. The mm/dd/yy are mapped � � � as follows: � � � 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 � � � y y y y y y y m m m m d d d d d � � � where: mm is 1-12 � � � dd is 1-31 � � � yy is 0-119 (1980-2099) � 쳐컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 22-31 � Reserved for system use. � 쳐컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 32 � Current relative record number (0-127) within the current block. � � � (See above). You must set this field before doing sequential � � � read/write operations to the diskette. This field is not initialized � � � by the open function call. � � � If the record size is less than 64 bytes, both words are used. � � � Otherwise, only the first 3 bytes are used. Note that if you use the � � � File Control Block at 5Ch in the program segment, the last byte of � � � the FCB overlaps the first byte of the unformatted parameter area. � 읕컴컴컴좔컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸 note 1) An unopened FCB consists of the FCB prefix (if used), drive number, and filename.ext properly filled in. An open FCB is one in which the remaining fields have been filled in by the CREAT or OPEN function calls. 2) Bytes 0-5 and 32-36 must be set by the user program. Bytes 16-31 are set by DOS and must not be changed by user programs. 3) All word fields are stored with the least significant byte first. For example, a record length of 128 is stored as 80h at offset 14, and 00h at offset 15. EXTENDED FILE CONTROL BLOCK The extended file control block is used to create or search for files in the disk directory that have special attributes. It adds a 7 byte prefix to the FCB, formatted as follows: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 � E X T E N D E D F I L E C O N T R O L B L O C K � 쳐컴컴컴쩡컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � Bytes � Function � 쳐컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 0 � Flag byte containing 0FFh to indicate an extended FCB. � 쳐컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 1-6 � Reserved � 쳐컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴캑 � 6-7 � Attribute byte. Refer to function call 11h (search first) for � � � details on using the attribute bits during directory searches. This � � � function is present to allow applications to define their own files � � � as hidden (and thereby excluded from normal directory searches) and � � � to allow selective directory searches. � 읕컴컴컴좔컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸 Any reference in the DOS function calls to an FCB, whether opened or unopened, may use either a normal or extended FCB. If you are using an extended FCB, the appropriate register should be set to the first byte of the prefix, rather than the drive-number field. MEMORY CONTROL BLOCKS DOS keeps track of allocated and availible memory blocks, and provides three function calls for application programs to communicate their memory needs to DOS. These calls are 48h to allocate a memory block, 49h to free a previously allocated memory block, and 4Ah (SETBLOCK) to change the size of an allocated memory block. CONTROL BLOCK DOS manages memory as follows: DOS build a control block for each block of memory, whether free or allocated. For example, if a program issues an "allocate" (48h), DOS locates a block of free memory that satisfies the request, and then "carves" the requested memory out of that block. The requesting program is passed the location of the first byte of the block that was allocated for it - a memory management control block, describing the allocated block, has been built for the allocated block and a second memory management control block describes the amount of space left in the original free block of memory. When you do a setblock to shrink an allocated block, DOS builds a memory management control block for the area being freed and adds it to the chain of control blocks. Thus, any program that changed memory that is not allocated to it stands a chance of destroying a DOS memory management control block. This causes unpredictable results that don't show up until an activity is performed where DOS uses its chain of control blocks. The normal result is a memory allocation error, which means a system reset will be required. When a program (command or application program) is to be loaded, DOS uses the EXEC function call 4Bh to perform the loading. This is the same function call that is availible to applications programs for loading other programs. This function call has two options: Function 00h, to load and execute a program (this is what the command processor uses to load and execute external commands) Function 03h, to load an overlay (program) without executing it. Although both functions perform their loading in the same way (relocation is performed for EXE files) their handling of memory management is different. FUNCTION 0: For function 0 to load and execute a program, EXEC first allocates the largest availible block of memory (the new program's PSP will be at offset 0 in that block). Then EXEC loads the program. Thus, in most cases, the new program owns all the memory from its PSP to the end of memory, including memory occupied by the transient parent of COMMAND.COM. If the program were to issue its own EXEC function call to load and execute another program, the request would fail because no availible memory exists to load the new program into. NOTE: For EXE programs, the amount of memory allocated is the size of the program's memory image plus the value in the MAX_ALLOC field of the file's header (offset 0Ch, if that much memory is availible. If not, EXEC allocates the size of the program's memory image plus the value in the MIN_ALLOC field in the header (offset 0Ah). These fields are set by the Linker). A well-behaved program uses the SETBLOCK function call when it receives control, to shrink its allocated memory block down to the size it really needs. A COM program should remember to set up its own stack before doing the SETBLOCK, since it is likely that the default stack supplied by DOS lies in the area of memory being used. This frees unneeded memory, which can be used for loading other programs. If the program requires additional memory during processing, it can obtain the memory using the allocate function call and later free it using the free memory function call. When a program is loaded using EXEC function call 00h exits, its initial allocation block (the block beginning with its PSP) is automatically freed before the calling program regains control. It is the responsibility of all programs to free any memory they allocate before exiting to the calling program. FUNCTION 3: For function 3, to load an overlay, no PSP is built and EXEC assumes the calling program has already allocated memory to load the new program into - it will NOT allocate memory for it. Thus the calling program should either allow for the loading of overlays when it determines the amount of memory to keep when issuing the SETBLOCK call, or should initially free as much memory as possible. The calling program should then allocate a block (based on the size of the program to be loaded) to hold the program that will be loaded using the "load overlay" call. Note that "load overlay" does not check to see if the calling program actually owns the memory block it has been instructed to load into - it assumes the calling program has followed the rules. If the calling program does not own the memory into which the overlay is being loaded, there is a chance the program being loaded will overlay one of the control blocks that DOS uses to keep track of memory blocks. Programs loaded using function 3 should not issue any SETBLOCK calls since they don't own the memory they are operating in. (This memory is owned by the calling program) Because programs loaded using function 3 are given control directly by (and return contrrol directly to) the calling program, no memory is automatically freed when the called program exits. It is up to the calling program to determine the disposition of the memory that had been occupied by the exiting program. Note that if the exiting program had itself allocated any memory, it is responsible for freeing that memory before exiting. MEMORY CONTROL BLOCKS Only the first 5 bytes of the memory control block are used. The first byte will always have the value of 4Dh or 5Ah. The value 5Ah indicates the block is the last in a chain, all memory above it is unused. 4Dh means that the block is intermediate in a chain, the memory above it belongs to the next program or to DOS. The next two bytes hold the PSP segment address of the program that owns the corresponding block of memory. A value of 0 means the block is free to be claimed, any other value represents a segment address. Bytes 3 and 4 indicate the size in paragraphs of the memory block. If you know the address of the first block, you can find the next block by adding the length of the memory block plus 1 to the segment address of the control block. Finding the first block can be difficult, as this varies according to the DOS version and the configuration. The remaining 11 bytes are not currently used by DOS, and may contain "trash" characters left in memory from previous applications. If DOS determines that the allocation chain of memory control blocks has been corrupted, it will halt the system and display the message "Memory Allocation Error", and the system will halt, requiring a reboot. Each memory block consists of a signature byte (4Dh or 5Ah) then a word which is the PSP value of the owner of the block (which allocated it), followed by a word which is the size in paragraphs of the block. The last block has a signature of 5Ah. All others have 4Dh. If the owner is 0000 then the block is free. User memory is allocated from the lowest end of available memory that will satisfy the request for memory.