Job File Table: Difference between revisions

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{{UnreferencedMore references|date=December 2009}}

The '''Job File Table''' (JFT) is a [[DOS]] data structure in the [[Program Segment Prefix]] (PSP). It starts at PSP offset 0x18 and is 20 bytes long. For each open file handle, [[DOS]] stores the index into the [[System File Table]] (SFT). A file handle that is returned by <code>open</code>, <code>_open</code>, and other similar functions is simply an index into the JFT where [[DOS]] stored the SFT entry index for the file or device that the program opened.

{{Orphan|date=December 2009}}

When a program starts, the first 5 entries in the JFT are preconnected to the standard devices. All other handles are marked as closed by setting the value of the entry to 0xFF. The standard devices are initialised as follows:

The '''Job File Table''' (JFT) is a [[DOS]] data structure in the [[Program Segment Prefix]] (PSP). It starts at PSP offset 0x18 and is 20 bytes long. For each open file handle, [[DOS]] stores the index into the [[System File Table]] (SFT). A file handle that is returned by open, _open and other similar functions is simply an index into the JFT where [[DOS]] stored the SFT entry index for the file or device that the program opened.

{| class="wikitable"

! Handle

Handle ! Description

|-

| 0

| [[stdin|Standard input]]

|-

| 1

| [[stdout|Standard output]]

|-

| 2

| [[stderr|Standard error]]

|-

| 3

| [[COM1]]

|-

| 4

| [[LPT1]]

|}<ref>{{Cite web|url=https://en.wikibooks.org/wiki/First_steps_towards_system_programming_under_MS-DOS_7/Appendix#A.01-4_System_File_Table_structure|title = First steps towards system programming under MS-DOS 7/Appendix - Wikibooks, open books for an open world}}</ref>

When a program starts, the first 5 entries in the JFT are preconnected to the standard devices. All other handles are marked as closed by setting the value of the entry to 0xFF.

The standard devices are initialised as follows

Handle Description

0 standard input

1 standard output

2 standard error

Handles 0-2 are connected to the console device. During program execution they can get modified (closed, redirected) like any other handle.

Because the size of the JFT in the PSP is limited to 20 bytes, you could originally only open 15 files (20- minus the 5 standard devices). Ascould be open at a time. ofIn MS-DOS 2.0, the Extended Job File Table was introduced, which allowed to open up to 254 files to be opened.

As of MS-DOS 3.30, the size of the JFT can be modified with a call to <code>INT 21h, 67h</code>.

==References==

As of MS-DOS 3.30 the size of the JFT can be modified with a call to INT 21h, 67h.

<references />

[[Category:DOS technology]]