Percom DoubleDOS 4.24 for the Model I - SYS3/SYS Disassembled

SYS3/SYS

Other Navigation

Percom DoubleDOS 4.24 SYS3/SYS Disassembly (Model I)

SYS3/SYS is the overlay of DoubleDOS (DBLDOS in Percom's manual) that closes and kills files. It is overlay 5 (directory record 05H): the overlay dispatcher of SYS0/SYS loads it at 4E00H when a request code with the low four bits 5 is given and calls its transfer address 4E00H with the code in Register A.

The file is 786 bytes: a name record, the copyright record of 169 bytes, one load block, 4E00H-504EH (591 bytes), and the transfer address 4E00H. Every loaded byte is on exactly one row below. SYS3/SYS is the same in DoubleDOS 4.20, 4.23 (all three sets) and 4.24.

Load Map

The Request Codes

How a File Is Closed

Differences From the Manual

Percom's manual has nothing on closing or killing files; it refers to the TRSDOS manual for the commands.

Notes

SYS3/SYS's Own Bytes

Memory Outside SYS3/SYS

The Directory Entry as SYS3/SYS Changes It

Error Codes Returned by SYS3/SYS

Outside Routines

4E00H - The Request Code Dispatcher

The transfer address of SYS3/SYS. The dispatcher of SYS0/SYS (4BA2H) calls it with the request code in Register A and the FCB in Register Pair DE: bits 6-4 choose the job, the low four bits (5) name SYS3/SYS. 95H closes a file, A5H kills it.

4E0DH - Close a File

Reached for request code 95H (4428H) with Register Pair DE the FCB. A changed buffer is written, the end of file is put in the directory entry when it differs, granules beyond the end of file are given back to the GAT (and an extra entry left with no extents is freed), and the FCB is written over with the file's specification as text, NAME/EXT:D, which also marks it closed. Returns Register A 00H and the Z FLAG, or the NZ FLAG and an error code: 26H the FCB is not open, 11H, 12H, 14H, 15H, 16H or 17H a read or write error.

4E96H - Give Back the Last Granule

Entered from 4F13H with Register Pair HL the granule byte of the last extent and Register Pair DE the granules still to give back. The last granule of the extent is marked free in the GAT at 4D00H and the extent's count made one less; an extent left with no granule becomes FFH FFH and the extent before it is the last. When an extra entry is left with no extent it is freed, its place in the hash index table cleared, and the entry before it ended with FFH where it had FEH.

4F1DH - Put the Specification Back in the FCB

Reached when the directory and the GAT are up to date. The name and extension of the file's first entry and its drive are written into the FCB as NAME/EXT:D and 03H; the password is not. The first byte, a letter, clears bit 7 of FCB+00H, so the FCB is no longer open, and it can be opened again as it is. When the file has an extra entry in another sector than its first, the sector read is that of Register B, the last entry read, and the name copied is the bytes at the first entry's place in that sector.

4F72H - Kill a File

Reached for request code A5H (442CH) with Register Pair DE the FCB of an open file. The access level must be 0 or 1. The granules of every extent of the file's first entry and of its extra entries are marked free in the GAT, each entry is freed (attribute 00H) and its place in the hash index table cleared; then the GAT and the hash index table are written and the 32 bytes of the FCB cleared. Returns Register A 00H and the Z FLAG, or the NZ FLAG and an error code: 26H the FCB is not open, 25H access denied, 11H, 12H, 14H, 15H, 16H or 17H a read or write error.

4FCEH - Free the Granules of an Extent

Called from 4FA1H with Register E the block and Register D the granule byte of an extent (bits 7-5 the first granule in the block, bits 4-0 the count less 1). Each granule's bit is reset in the GAT at 4D00H, two granules a block.

4FF0H - Reset a Bit of Register B

Called with Register B a GAT byte and Register A the number of the bit (the low three bits). The routine writes the instruction RES n,B at 4FFAH and runs it.

4FFDH - Read the Hash Index Table

Called with Register C the drive. Sector 1 of the drive's directory block, the hash index table, is read into 5100H, after SYS3/SYS, since the GAT is at 4D00H. 5033H keeps Register Pairs BC and DE for it. Returns the Z FLAG, or error 16H.

500CH - Write the Hash Index Table

Called with Register C the drive. The table at 5100H is written back as sector 1 of the drive's directory block with the deleted data mark and read back. Returns the Z FLAG, or error 17H.

501BH - Divide, Rounded Down When Exact

Called from 4E80H with Register Pair HL the number of sectors and Register A 05H. Returns Register Pair HL the quotient, or the quotient less 1 when the remainder is 0, so that one more is the number of granules needed.

5022H - The Number of Sectors, Then Read the Entry

Called from 4E19H with Register B the record and Register C the drive. FCB+0CH-+0DH, the end of file sector, is made the number of sectors again when the last sector is partly filled (SYS2/SYS made it one less, 5180H there). Then the directory entry is read.

5033H - Keep Register Pairs BC and DE Around the Rest of a Routine

Called as the first instruction of 4FFDH and 500CH. The return address, the rest of the routine, is taken off the stack; Register Pairs DE and BC are saved; the rest of the routine is called through 4B55H, which loads Register D with the drive's directory block; when it returns, Register Pairs BC and DE are restored and 5033H returns to the routine's caller.

5040H - Step Back and Write the Entry

Called from 4F08H with Register Pair HL entry +1EH of the entry before a freed extra entry. Register Pair HL is made entry +1DH, the granule byte of that entry's last extent, and the sector is written.

5047H - Read the GAT, Then Give Back Granules

Reached from 4E92H with Register Pair HL the granule byte of the last extent and Register Pair DE the granules to give back.