LDOS 5.3.1 for the Model III - SYS12/SYS Disassembled

SYS12/SYS

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LDOS 5.3.1 SYS12/SYS Disassembly - @DODIR and @RAMDIR (Model III)

SYS12/SYS holds the two directory services that programs call: @DODIR (4419H, request AEH) shows the files of a drive on the screen or lists their directory records into a buffer, and @RAMDIR (4290H, request 9EH) puts a short record of each file, or of one file, or the space used and free, into a buffer. The manual says the MiniDOS filter uses them and that other programs that show a directory may. The supervisor dispatcher loads SYS12/SYS into the overlay area at 4E00H and calls it with the request code in Register A (4E00H).

The file is a load module of 839 bytes: a record of type 1FH with the text Copyright 1991 MISOSYS, Inc., All rights reserved, three load blocks for 4E00H-50FDH, a load block for the two bytes 4BC8H-4BC9H (02FEH, the 766 bytes that SYS12/SYS takes up in the overlay area) and the transfer address record for 4E00H.

The Calls

The Walk through the Directory

About This Page

4BC8H - The Overlay Length (Data)

Every system file from SYS1/SYS to SYS12/SYS carries a load block for the two bytes 4BC8H-4BC9H in the resident area of SYS0/SYS. They hold the length of the file's part in the overlay area: for SYS12/SYS 02FEH, the 766 bytes 4E00H-50FDH. In the file this block is the last load block, after the three blocks of 4E00H-50FDH and before the transfer address record (4E00H). The two bytes are data, not instructions.

4E00H - The Two Requests, and @RAMDIR

The supervisor dispatcher loads SYS12/SYS into 4E00H and calls it with the request code in Register A. Bits 6-4 select the service: 20H, request AEH from @DODIR (4419H), goes to 4EE6H; 10H, request 9EH from @RAMDIR (4290H), is handled here; any other function only loads the file. @RAMDIR takes Register B = the drive (0-7, else error 20H), Register C = what is wanted and Register Pair HL = a buffer. Both services enter through the file prologue of SYS0/SYS at 49F1H, so their RET goes through 4A14H, which gives the caller back its Register Pairs BC, DE and HL and Index Register IX: the results are in the buffer, Register A and the flags. With Register C = FFH the buffer gets the space of the diskette in K (1024 bytes): the space used, then the space free (4E1DH). With Register C = 00H it gets a record of 22 bytes for each visible file (4EA6H); any other Register C names directory record C less 1, and the buffer gets that file's record (4E3AH).

4E3AH - @RAMDIR: One File's Record

Register B = what was asked, plus one. 00H (now 01H) asks for every file (4EA6H); any other value is the number of a directory record plus one. The record's sector (bits 4-0 of its number) must lie on the directory cylinder below the sectors that hold records (the sectors of a cylinder less 2, for the allocation table and the hash index table), else error 10H; the record must be in use, primary, not a system file and not invisible (bits 7, 6, 4 and 3 of its attributes 10H only), else error 19H. The record built at 4E77H for the buffer is 22 bytes: +00H-+0EH the file's name as NAME/EXT:d, padded with spaces; +0FH its protection level (bits 2-0 of the attributes); +10H its end of file byte; +11H its record length; +12H-+13H its ending record number; +14H-+15H its size in K (that number divided by 4); then 2BH at +16H, which the next record overwrites.

4EA6H - @RAMDIR: Every Visible File

The hash index table (sector 1 of the directory cylinder) is read into 5100H, and its bytes are taken in the order of the directory: row by row down each column (L + 20H), then the next column, up to 32 columns; a byte of 00H is a free record. For each record in use its directory sector is read into 4300H, unless the sector number matches the one kept at 4ECAH, the operand of the CP at 4EC9H; a primary record in use that is neither a system file nor invisible gets its 22 byte record in the buffer (4E77H). The operand is FFH only in the file: when @RAMDIR is called again while SYS12/SYS is still in the overlay area, the first record whose sector equals the one kept is taken from 4300H without reading it. The record of the single file at 4E6AH does not change the operand.

4EE6H - @DODIR

Request AEH, issued by @DODIR (4419H). Register C = the drive (0-7, else error 20H), Register B = the function, Register Pair HL = a buffer whose first three bytes are a mask for the extension. The function is kept at 4F6CH, the operand of LD A at 4F6BH. Function 04H puts the diskette's name and date (allocation table +D0H-+DFH) and its capacity and free space in K into 20 bytes of the buffer (4F1AH). Functions above 04H give error 1DH. Functions 00H-03H go through the files of the directory like @RAMDIR, from 4F2CH: bit 0 set puts 18 bytes of each file's directory record into the buffer (+00H-+0FH and +14H-+15H) and ends the list with FFH; bit 0 clear shows NAME/EXT:d of each visible file on the screen, four to a line, and waits for a key and clears the screen after 15 lines. Bit 1 set shows only the files whose extension matches the mask (4FC4H). The buffer list (bit 0) takes the system files and the invisible files too; the screen list does not.

4F2CH - @DODIR: Go through the Files

The same walk through the hash index table at 5100H as @RAMDIR's (4EADH), with the sector kept at 4F5AH, the operand of the CP at 4F59H (FFH only in the file, as at 4ECAH). For each record in use the address 4F2CH is pushed as the return address, so that the handler's RET, or a RET NZ for a file passed over, goes on to the next record. At the end the buffer list gets FFH, the screen list a new line.

4FC4H - Test the Extension against the Mask

Register A = the function of @DODIR, Register Pair HL = the directory record. When bit 1 is clear every file passes (the Z FLAG). Else the three characters of the extension (+0DH-+0FH) are compared with the mask at 4485H-4487H: a dollar sign in the mask matches any character, and a mask character of 41H or above is made a capital (bit 5 cleared) first. The NZ FLAG on the first difference.

4FE2H - Write NAME/EXT:d

Register Pair HL = a directory record (+00H), Register Pair DE = the destination. The name (+05H-+0CH) is copied up to its first space, then a slash and the extension (+0DH-+0FH) up to its first space when it has one, then a colon and the drive's digit (the operand at 5017H, stored by 50B1H), then spaces up to 15 bytes in all, and 03H. Register C counts the 13 places for the name, the slash and the extension. Register Pair DE ends at the 03H.

5025H - The Capacity and the Free Space in K

Register Pair HL = the place for four bytes, Register C = the drive. The granule allocation table is read into 5100H (508BH). The capacity in sectors is the cylinders (byte +06H of the drive code table plus one) times the sectors of a cylinder (the sectors of a granule, bits 4-0 of +08H plus one, times the granules of a track, bits 7-5 plus one, doubled when bit 5 of +04H says the drive has two sides). The free space in sectors is the free granules times the sectors of a granule. The free granules are counted in the table from 5100H for as many cylinders as the byte at 51CCH (allocation table +CCH) plus 23H (35): each byte is shifted right with a 1 coming in at the top until it is FFH, and every 0 bit shifted out is a free granule. 5075H turns a count of sectors into K, (sectors + 2) / 4 from the low 16 bits of the @MULT product, and stores it: the capacity at +00H-+01H, the free space at +02H-+03H. Register Pair HL ends at +03H and Register Pair DE holds the free space in K.

508BH - Read the Allocation Table or the Hash Index Table

508BH reads sector 0 of the directory cylinder, the granule allocation table, into 5100H (error 14H); 509CH reads sector 1, the hash index table, into 5100H (error 16H) and returns Register Pair HL = 5100H. Both use RDSSEC, which accepts only a sector with the deleted data mark, and return the Z FLAG on success.

50AEH - Is the Drive There and Turning?

Register C = the drive. Its digit (30H + the drive) is stored at 5017H for the name builder. The drive's code table must start with C3H (a JP to its driver); otherwise the NZ FLAG is returned with Register A = that byte less C3H (06H for the C9H of a drive that is not present). The drive is selected by a seek to the cylinder the head is on and the controller waited for. Unless bit 4 of byte +04H of the code table is set, the index pulse (bit 1 of the controller's status) must then go off, on and off before TIMER$ (4288H, 30 ticks a second on the Model III) has counted 15 more ticks: else error 08H, device not available, with the NZ FLAG. 50ECH reads the status once and gives up at the deadline by dropping its caller's return address and going to 50E8H. Index Register IY and Register Pair HL are kept.