BDOS interrupts - INT E0h

If a two-byte program return code is to be set, a prior call to function 6Ch should be made.

This call is exactly the same as function 8Fh.

If printer output is active (CTRL-P), characters are also sent to the list device, PRN. if no input character is ready the call waits until a character is returned.

If the call is made with DL = 0FDh and no character is ready the system waits for input.

If DX is set to 0FFFFh on entry the default DMA buffer is used, any other value points to a specified buffer offset.

All disc drives are set to read/write, the default drive is set to A: and the default DMA address is set to 080h relative to the current DMA segment address (i.e, the default in the PSP).

If an error occurs and the filing system is in default error mode, a message is displayed and the call terminates, otherwise AL is set to 0FFh and AH contains the error code.

If a file is opened for write access the access date and time stamps are updated, if applicable. If there is no physical error but the file does not exist, AL is set to 0FFh and AH is set to zero.

The '?' wildcard is permitted in any filename character position or in the file type specification. If the '?' wildcard is placed in the drive field, the current drive and the current user number are searched. if wildcards are specified in the file's name or type, the first matching name is returned.

If the call is successful, the file's directory data is transferred to the FCB. If there is no physical error but the file does not exist, AL is set to 0FFh and AH is set to zero.

F5 interface attribute :0 = standard delete
1 = delete only FCBs

If there is no physical error but the filename and type are not matched AL is set to 0FFh and AH is set to zero. For all operations, if any matching file fails the password check or is set to read only, the file (and its FCB) are not deleted.

On reading the CR field is updated to the next record position. If the CR field overflows, the system automatically opens the next extent and resets CR to zero for the next read operation. Codes of 1, 9 or 0Ah may be returned on physical error.

On writing, the CR field is updated to the next record position. If the CR field overflows, the system automatically opens the next extent and resets CR to zero for the next write operation. Codes of 1, 2, 9 or 0Ah are returned on physical error.

If the file password is to be used, F6 is set to one and the password must be placed in the first eight bytes of the DMA buffer and byte nine of the DMA buffer set to password mode. A new file entry in the FCB is created and an extended FCB is created if password mode is set on a successful call. An file attributes are set to zero.

If there is no physical error but AL is returned as there is no space in the directory.

If the file is password protected, the password must be placed in the first eight bytes of the correct DMA buffer, or the password must have been previously set as the default password.
If there is no physical error but AL is returned as 0FFh, the named old file does not exist.

Each bit which is set to one represents a currently logged-in drive. A logged-in drive has had its directory read into memory and its allocation vectors built. A drive can be logged-in explicitly by a call to function 0Eh, or implicitly by a system or manual file operation which requires access to the drive.

This function should be called before using any disc access functions if the command tail is to be preserved.

Caution: Note that, although this call is grouped with drive functions, it really acts on the current disc in the current drive. That is to say, the setting is not permanent and can be amended by simply changing the disc and issuing any disc access command, including a manual 'DIR'.

If failed: AL = return code, AH = physical error (1, 2, 4, 7 or 9)

If a physical error is encountered AH contains the actual number of records read before the failure.

The file may be read sequentially after this call, but the current record field in the FCB is not advanced by the function, therefore a switch to sequential access will re-read the same record if the CR field is not changed.

If the file has been written exclusively in sequential mode the returned value is the same as the physical number of records in the file plus one. Note, though, that if the file has been written randomly, the record number sequence may well contain gaps. In this case the returned value will not give an accurate indication of the number of records in the file, but instead it gives the logical file size as a potential record count.

In default mode an error is displayed and the action or program is terminated.

The file will be searched for in the current and P: drives, under the current user and user zero. if the file is found the current program is terminated, its memory released and the new program is loaded and executed. If there is insufficient memory for the new program, because the calling program is already terminated, loading of the new program is abandoned and a message is output to the console device.

A two byte code may be passed from the caller to the new routine by means of function 6Ch.

If DL is set to 0FFh all the calling routines internal record buffers are also purged. This is necessary if read-after- write verification is required, to ensure that the verification data is taken from disc rather than from the program's record buffers.

0 Number of the system variable(s)
1: 0 = get variables, 0Fh = set variables
2-7 (1-5 bytes) The value of the system variable

SCUPB updated if 'get variables'

The memory block parameters shown as CX and DX should contain the 16 bit values passed in those registers when normal BDOS calls are made.

As can be seen these functions have direct equivalents in BDOS calls and several similar facilities are also available via DOS interrupt 21h.

These calls therefore offer no advantage over BDOS or DOS calls, given that they are more difficult to implement.

The layout of the MCB is as follows (bytes):
0-1: MCB segment, start of memory
2-3: MCB length, length of allocation
4: MCB extent, additional param./returned byte value.

For this call the MCB segment is set to zero. The MCB length is set to the minimum number of paragraphs required. MCB extent is set to zero if the memory is to be released after program termination, or 2 if the allocation is to be retained after termination.

Caution. This call allocates the whole of the largest single memory block in the system, not limited to the amount requested.

However, all the remainder of that block is allocated, not limited to the amount specified.

MCB segment = Start paragraph required
MCB length = Exact No. of paragraphs required
MCB extent = 0 if area is to be released
= 1 if allocation is to be retained

The preference for function 38h is because in a single user system the smallest available memory block is also often the only one, extending to the top of RAM. if any of the preceding calls are used and MCB extent is set to two, it may be that no other program can be run subsequently, even after termination of the current task, as the entire memory may remain allocated.

MCB segment = Start of allocated memory
MCB length = No. of paragraphs to release
MCB extent = 0 if the specified part of the allocation is to be released
= 0FFh if the entire allocation is to be released

It is not possible to release a block in the middle of an allocation, or which includes the start but does not extend to the end.

Note, the program is not called for execution by this call. If the loaded module is an RSX, both AX and BX are set to zero.

If the file is not found, the truncation length is not less than the current file length, or no such record number exists in the file (as it may not in random files) the call returns an error.

If an error occurs and the filing system is in default error mode, a message is displayed and the call terminates, otherwise AL is set to 0FFh and AH contains the error code.

If the time stamp fields are set to binary zeros date/time stamping is not enabled.

If a file so marked is opened for write access the date and time stamps are updated if applicable.

If there is no physical error but the file does not exist, AL is Set to 0FFh and AH is set to zero.

If the file is password protected the correct password must be placed in the first eight bytes of the current DMA. If bit 0 of the extent field is zero (new password) the new password must be placed in the second eight bytes of the DMA.

If there is no physical error but the file does not exist, AL is set to 0FFh and AH is Set to zero.

The return code can therefore be used by a calling program, or by the batch file command
IF ERRORLEVEL

Return codes are:

A default return code of zero is set by the system unless the program was loaded by a previous program, by means of function 2Fh.

The console mode variable is a 16-bit word with significant values as follows:

If the console is not in default mode the actions are determined by the current mode setting.

The PFCB contains the offset of the ASCII file specification, followed by the offset of the target FCB to be filled by the system. A minimum length string of 128 bytes is permitted, while the length of the target FCB must be 32 bytes. The filename is in the format:

(d:)filename{.ext} {;password}

where each of the items enclosed by { } is optional.

The system regards any parsing delimiter in the input string which is out of context with its location in the file specification as the end of the string.

The following are recognised as string delimiters, unless they are logically appropriate to their location in the ASCII file specification:

00h   null
0Ch   tab
0Dh   return
20h   space
2Ch   , (comma)
2Fh   | (solidus)
2Eh   . (period)
3Ah   : (colon)
3Bh   ; (semicolon)
30h   < (less than)
3Dh   = (equal)
3Eh   > (greater than)
5Bh   [ (left square bracket)
5Dh   ] (right square bracket)

All filenames and extensions are written to the FCB in upper case and blanks are used to pad the filename to eight characters and the extension to three as required. If a '*' occurs in a filename, that character and all remaining characters in the FCB entry are set to '?'.

It should be noted that the location of SYSDAT data is not fixed and may vary from version to version of DOS Plus, therefore access to this area should be used with discretion and only when absolutely necessary.

If no data is ready when the call is issued it waits until at least one character has been received. If "input exhausted" then occurs before the specified number of characters have been read, the call returns immediately

If no data can be sent when the call is issued it waits until at least one character can be transmitted. If "output full" then occurs before all the specified data has been sent the call returns immediately.