GeneveOS Device Operation

Accessing devices (floppy disk, hard disk, serial connector, printer) is possible in GeneveOS via XOP calls.

User-task XOPs

User-task XOPs are available for use in application programs. Here is a typical example:

PABADD EQU  >F180   
FILE   DATA 8
...
       LI   R0,PABADD
       XOP  @FILE,0
       MOVB @PABADD+2,R0
       JNE  ERROR
       ...

Similar as with the TI-99/4A device service routine concept (DSR), a Peripheral Access Block (PAB) must be set up prior to invoking the XOP.

Device Service Routine Call

	Input	Output
R0	Pointer to PAB	-

The actual operation is contained in the Peripheral Access Block (PAB).

Peripheral Access Block

While the TI-99/4A DSRs expect the PAB to be stored in VDP RAM, the Geneve OS DSRs use CPU RAM for the PAB, which means the PAB need not be copied to the video RAM before use. Also, you can choose to have the I/O buffers in VDP or CPU RAM.

The general layout of the PAB is as follows:

00	01	02	03	04	05	06	07	08	09	0A	0B	0C	0D	0E	0F	10 ...
Opcode	Mode	Error code	Buffer address			Record number		Record length		Memory type	Character count			Status byte	Name length	Name

Note that the buffer addresses are 21-bit wide, represented by the virtual address within the calling task. The address for this buffer should be derived from memory allocation operations that were invoked earlier.

After the operation, the PAB contains information about the results of the operation.

Operations

OPEN

Open a record-oriented file. A file must be opened before it can be read from or written to. This operation is not required for flat memory image files.

The PAB must be set up prior to invoking the operation:

IN

00	01	02	03	04	05	06	07	08	09	0A	0B	0C	0D	0E	0F	10 ...
00	Mode	-	-			Records to reserve		Record length		-	-			-	Name length	Name

Open new file: The Record Length must be set to a value greater than 0. This will become the length of the records of the new file.

Open existing file: The Record Length must match the length of the records. If the length is unknown, 0 may be passed.

After the OPEN operation has completed, the PAB delivers information about the result:

OUT

00	01	02	03	04	05	06	07	08	09	0A	0B	0C	0D	0E	0F	10 ...
-	-	Error code	-			-		Actual Record Length		-	0	True Record Length		-	Name length	Name

The Actual Record Length is the length of the records of the existing file. When 0 was passed on the call, this delivers the record length of the file; otherwise this is the value that was passed for the call. The True Record Length is the record length of the file when it already exists.

CLOSE

Closes a record-oriented file. A file must be closed after being used. After closing, no further read or write operations may be invoked on the file. This operation is not required for flat memory image files.

The PAB must be set up prior to invoking the operation:

IN

00	01	02	03	04	05	06	07	08	09	0A	0B	0C	0D	0E	0F	10 ...
01	Mode	-	-			-		-		-	-			-	Name length	Name

After the CLOSE operation has completed, the PAB delivers information about the result:

OUT

00	01	02	03	04	05	06	07	08	09	0A	0B	0C	0D	0E	0F	10 ...
-	-	Error code	-			-		-		-	-			-	Name length	Name

BWRITE

Writes a sector to a file or to a device. This is a level-2 operation, that is, it accesses the device based on sector addressing (or sectors of a file). BWRITE allows for three operations:

• Create a new, empty file.
• Write sectors to a file.
• Write sectors to a device.

IN

00	01	02	03	04	05	06	07	08	09	0A	0B	0C	0D	0E	0F	10 ...
0B	-	-	Buffer address			Sector offset		-		-	Number of sectors			-	Name length	Name

Create a file: When a file is addressed, and the number of sectors is 0, a new file is created. The contents of the file should be written by BWRITE on the same file with a sector count greater than 0.

Write to file: When a file is addressed, the operation writes a sequence of sectors to the file. The Sector offset is the number of the first sector of the file, counting from 0. The physical locations of the sectors are subject to the sector allocation on the device; in particular, when fragmentation is required, the sectors are written to the new fragment.

Writing to device: When a device is addressed, the operation writes a sequence of sectors to the device. The Sector offset is the physical sector number.

After the BWRITE operation has completed, the PAB delivers information about the result:

OUT

00	01	02	03	04	05	06	07	08	09	0A	0B	0C	0D	0E	0F	10 ...
-	-	Error code	Updated buffer address			Next unwritten sector		-		-	0	Number of unwritten sectors		-	Name length	Name

The Updated buffer address points to the memory address after the sectors were written (e.g. when 4 sectors are written, the pointer is increased by 4*256 = 1024).

The Next unwritten sector is the first sector that has not been written due to an error condition.

The Number of unwritten sectors is the number of unwritten sectors due to an error condition.

Unless you want to write directly to the device, make sure that the PAB contains a file name that points to a file, not to a device. For example, when the file name has been created by the program, and for some reason the file name is empty (like "HDS1."), the following write operation will overwrite the volume information block and the allocation tables and destroy the target device filesystem.

Tick here: [ ] Yes, I fully understood the danger of this operation.

(The author lost a complete hard disk by this incident, and was lucky to have a backup.)