LDOS 5.3.1 for the Model III - SYS10/SYS Disassembled

SYS10/SYS

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LDOS 5.3.1 SYS10/SYS Disassembly - @KILL and the Module Search (Model III)

SYS10/SYS carries two services. The supervisor dispatcher loads it into the overlay area at 4E00H and calls it with the request code in Register A, whose bits 6-4 choose the service (4E00H): request 9CH, issued by @KILL (442CH), deletes the file whose control block is at Register Pair DE, and request BCH finds a module kept in memory above HIGH$ by its name (4F28H). The manual says SYS10/SYS holds the procedures to delete a file and should stay on working system diskettes.

The file is a load module of 440 bytes: a record of type 1FH with the text Copyright 1991 MISOSYS, Inc., All rights reserved, two load blocks for 4E00H-4F72H, a load block for the two bytes 4BC8H-4BC9H (0173H, the 371 bytes that SYS10/SYS takes up in the overlay area) and the transfer address record for 4E00H.

How a File Is Deleted

The Module Search

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 SYS10/SYS 0173H, the 371 bytes 4E00H-4F72H. In the file this block is the last load block, after the two blocks of 4E00H-4F72H and before the transfer address record (4E00H). The two bytes are data, not instructions.

4E00H - The Request Codes, and @KILL: Delete a File

The supervisor dispatcher loads SYS10/SYS into 4E00H and calls it with the request code in Register A. Bits 6-4 select the function: 30H (request BCH) is the search for a module above HIGH$ (4F28H); 10H (request 9CH, issued by @KILL at 442CH) deletes the file whose control block is at Register Pair DE; any other function only loads SYS10/SYS and returns with the NZ FLAG. A control block whose bits 7-5 of +00H are all clear is a device: it is closed (4E72H) and no disk is touched. For an open file the access granted (bits 2-0 of +01H) must be 0 or 1, else error 25H; then the granule allocation table is read into 5000H and each directory record of the file is freed (4E29H).

4E29H - Free the Directory Records of the File

For each directory record of the file, the primary record and then each extension record: the granules of its extents are freed in the table at 5000H (4E8BH); bit 4 of its attributes (the record in use) is cleared and the directory sector written back; its byte in the hash index table is set to 00H and the table written back. The cylinder byte that ends the record's extents (FEH or FFH) and the byte after it are kept in the operand at 4E5BH: FEH means the file goes on in the extension record whose number is that byte after it. After the last record the allocation table is written back and the 32 bytes of the control block are cleared.

4E72H - Close a Device

The control block at Register Pair DE is a device: +06H-+07H hold its two letter name. It is turned back into the device specification it was opened from: an asterisk (2AH), the two letters and the end byte 03H at +00H-+03H. No disk is touched.

4E8BH - Free the Granules of One Extent

Register E = the extent's cylinder, Register D = its granule byte (bits 7-5 the first granule on the cylinder, bits 4-0 the granules less one), Register C = the drive, and the allocation table in 5000H-50FFH (byte 5000H + c for cylinder c, bit n set while granule n is in use). The granules of a cylinder are bits 7-5 of byte +08H of the drive code table plus one, doubled when bit 5 of byte +04H says the drive has two sides; that count is stored at 4EBCH. Starting at the extent's first granule, each of its granules has its bit cleared; past the cylinder's last granule the count goes on at granule 0 of the next cylinder. Register Pairs HL and BC are kept.

4EC7H - Clear a Granule's Bit

Register A = the granule n (bits 2-0), Register B = an allocation table byte. Builds RES n,B at 4ED1H by storing 80H + 8n at its second byte 4ED2H and runs it: Register B comes back with bit n cleared.

4ED4H - Read or Write the Granule Allocation Table

Two entries one byte apart. 4ED4H is OR AFH, which leaves Register A non zero and so the NZ FLAG: read the table. Entered at 4ED5H, the byte AFH alone is XOR A, which sets the Z FLAG: write it. The flags are kept on the stack across DIRCYL and choose the work. The table is sector 0 of the directory cylinder of drive Register C, held at 5000H-50FFH: read with RDSSEC (error 14H, GAT read error), or written with the deleted data mark and verified (error 15H, GAT write error). Each returns the Z FLAG on success. Register Pairs DE and HL are kept.

4EF8H - Read and Write the Hash Index Table

The hash index table is sector 1 of the directory cylinder of drive Register C. 4EFDH reads it into 4300H (error 16H, HIT read error); 4EF8H would read it into 5000H instead, but nothing calls it. 4F0FH writes it back from 4300H with the deleted data mark and verifies it (error 17H, HIT write error). Each returns the Z FLAG on success and keeps Register Pairs BC and DE.

4F28H - Find a Module above HIGH$

Request BCH. Register Pair DE = a name, ended by any character below 21H (a space, ENTER, 03H). Programs that keep themselves in memory above HIGH$ (4411H) start with a header: a JR (18H) over it, the link word (the HIGH$ before the module was put there), a byte whose bits 3-0 are the length of the name, and the name. The newest module starts at HIGH$ + 1, and each link word + 1 is the start of the module before it, up to a link of FFFFH. Found: Register Pair HL = the module, Register Pair DE = the byte after its name, Register A = 00H and the Z FLAG. Not found (or no module): error 08H, device not available. The first module is taken at HIGH$ less the top of memory found by the reset (4415H), with the CARRY FLAG clear: that is HIGH$ + 1 only when the top is FFFFH (48K); with the top at 7FFFH or BFFFH it is another address, whose byte is not taken as a module unless it holds 18H. When HIGH$ equals that top, there is no module.