LDOS 5.3.1 for the Model III - SYS1/SYS Disassembled

SYS1/SYS

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LDOS 5.3.1 SYS1/SYS Disassembly - The Command Interpreter (Model III)

SYS1/SYS is the command interpreter of LDOS. It is a system file, loaded into the overlay area at 4E00H by the supervisor dispatcher of SYS0/SYS (4BCCH) whenever a request code names it: bits 3-0 of the code are 3. It displays LDOS Ready, reads a command line, and carries the command out, either as a member of one of the two command libraries, SYS6/SYS and SYS7/SYS, or as a program file loaded and run by @RUN. It also serves three supervisor calls that programs use to read their command lines: @FSPEC (a file specification), @FEXT (a default extension) and @PARAM (a parameter list).

The file is a load module of 1100 bytes: a record of type 1FH with the text Copyright 1991 MISOSYS, Inc., All rights reserved, four load blocks for 4E00H-51FEH, a load block for the two bytes 4BC8H-4BC9H (the length of the file's part at 4E00H, 03FFH) and the transfer address record for 4E00H. A library command runs at 5200H, above SYS1/SYS, so SYS1/SYS is still in memory while it runs: the LIB command reads the command table at 4EC3H from there. The first request of the command that loads another system file into 4E00H replaces it.

Functions

The Command Line

The BREAK Vector at 4203H

The Status Byte at 4758H

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 SYS1/SYS 03FFH, the 1023 bytes 4E00H-51FEH. In the file this block is the last load block, after the four blocks of 4E00H-51FEH and before the transfer address record (4E00H). The member of SYS7/SYS that is entered for SYSTEM reads and writes the word (its 55E4H, 5692H). The two bytes are data, not instructions.

4E00H - Entry: Pick the Function of the Request

The supervisor dispatcher (4BCCH) loads SYS1/SYS into 4E00H for a request code whose bits 3-0 are 3 and calls 4E00H with Register A = the request code (from OVRLY$, 4414H) and the caller's Register Pairs BC, DE and HL. Bits 6-4 of the code pick the function: 00H (83H) nothing; 10H (93H: @EXIT 402DH, @ABORT 4030H, @CMD 4296H) the LDOS Ready command loop; 20H (A3H, 429CH) carry out the command line at Register Pair HL and return to the caller; 30H (B3H, @CMNDI 4299H) carry out the command line at Register Pair HL, then LDOS Ready; 40H (C3H, @FSPEC 441CH) take a file specification; 50H (D3H, @FEXT 444BH) add a default extension; 60H (E3H, @PARAM 4454H) read a parameter list. Function 70H, which no vector of SYS0/SYS issues, is carried out as 20H.

4E2EH - The LDOS Ready Command Loop

Function 10H (request 93H): @EXIT (402DH), @ABORT (4030H) and @CMD (4296H) all lead here. The stack is set up afresh at 41E5H, the BREAK trap is armed when DEBUG is on, LDOS Ready is displayed and a line of up to 63 characters is read from the keyboard into INBUF$ (4225H) through the ROM's line input at 0040H. The command gets 402DH (@EXIT) as its return address, so when it ends the loop runs again.

4E5CH - Point the BREAK Vector at 4206H

Every command line, typed or given by a program, passes here before it is carried out. The jump at 4203H, which SYS0/SYS's reset sets to its cassette exit at 44FAH, is made a jump to 4206H. The ROM jumps to 4203H when BREAK is pressed while it waits on the cassette or the RS-232 port, and SYS0/SYS's interrupt routine sends interrupt bits 0 and 1 there. At 4206H-4208H SYS0/SYS holds three bytes of 00H, which run on into its @CKDRV request at 4209H. Nothing on this diskette puts 44FAH back at 4204H-4205H; BASIC/CMD stores an address of its own there (its 5C90H).

4E68H - Carry Out a Command Line

Register Pair HL = the command line (in INBUF$, 4225H). The line is first passed to the job log (428DH). A line that starts with a period is a comment (4EACH). A line that starts with ! is always run as a program file. Otherwise the first word is taken as a file specification into CFCB$ (4485H) and looked up in the table of library commands at 4EC3H; a command found there is carried out by its member of SYS6/SYS or SYS7/SYS (4EB4H). Any other word gets the extension /CMD when it has none and is loaded and run by @RUN (4433H), with Register Pair HL pointing at the rest of the line.

4EA3H - End a Command That Cannot Be Carried Out

Reached when BREAK ended the typed line, when the first word is not a file specification and when it is a device name. A command line from a program (function 20H, bit 1 of 4758H set) returns to the program with Register A = 02H and the NZ FLAG; otherwise @ABORT (4030H) ends it and LDOS Ready follows.

4EACH - A Comment Line

A command line that starts with a period is not carried out. While a JCL job runs (bit 5 of SFLAG$, 442BH) the command returns at once, so the job goes on with its next line; otherwise the next line is read from the keyboard without LDOS Ready being displayed again.

4EB4H - Carry Out a Library Command

The word was found in the table at 4EC3H: Register E = the entry's member number and Register D its flags. When bit 7 of the flags is clear, Register Pair DE is the address of the command's code in SYS1/SYS (only BOOT, 4EC2H). When it is set the command is a member of a library: of SYS6/SYS (request 88H) when bit 6 is clear, of SYS7/SYS (request 89H) when bit 6 is set. RST 28H loads member Register B and runs it with Register Pair HL at the parameters. SYS1/SYS stays in memory at 4E00H-51FEH while the member runs above it, at 5200H (5A00H for FILTER and SET).

4EC2H - BOOT: Start the System Again

The table entry of BOOT holds the address 4EC2H instead of a member number. RST 00H jumps to the ROM's power up entry at 0000H, which starts the computer as if it had just been turned on: the boot sector of drive 0 is read and LDOS is loaded again.

4EC3H - The Library Command Table (Data)

35 entries of 8 bytes, searched by 50D1H (called from 4E8EH) and listed by the LIB command (the member 19H of SYS6/SYS reads the table from 4EC3H while SYS1/SYS is still in memory). Each entry is the command name in capitals, padded with spaces to 6 characters, then two bytes: the member number and the flags. Flags 80H: a member of SYS6/SYS, request 88H; C0H: a member of SYS7/SYS, request 89H (bit 7 a library command, bit 6 SYS7/SYS). The two bytes of BOOT are the address 4EC2H. A 00H after the last entry ends the table (4FDBH). The member numbers are those of the type 08H member headers of SYS6/SYS and SYS7/SYS, which also give the address each member is entered at; several commands share the code of one member through different entry addresses. Member 1CH of SYS7/SYS has no entry here.

4FDCH - @FSPEC: Take a File Specification

Function 40H (request C3H, 441CH); the command loop calls it too (4E7BH). Register Pair HL = the text, Register Pair DE = where the specification goes (a file control block). It copies NAME/EXT.PASSWORD:D: a name of up to 8 letters and digits, then a / and an extension of up to 3, a period and a password of up to 8, a colon and a drive of 1. Small letters become capitals and characters past a field's length are dropped. Instead of a name the text may hold an asterisk and a device name of up to 2 letters and digits, such as the keyboard's KI. The specification ends with 03H. When the word taken is TO, ON, OVER or USING (the table at 502DH) it is passed over and the next word is taken. Returns Register A = 00H and the Z FLAG, Register Pair DE unchanged and Register Pair HL after the specification (past the character that ended it and any spaces after it, or at an ending 0DH, 03H or open parenthesis); or the NZ FLAG with Register A = the character found where a name, a password or a drive was needed.

501EH - A Device Name

The text did not start with a letter or a digit. When it starts with an asterisk, a device name of up to 2 letters and digits follows (KI, DO, PR); any other character is an error.

502DH - The Words @FSPEC Passes Over (Data)

Four entries in the layout of the command table, searched by 50D1H from 5016H: a word padded with spaces to 6 characters and two bytes, here always 4300H, which @FSPEC does not use. A 00H ends the table. In a line such as FILE1 TO FILE2, a second call of @FSPEC after FILE1 takes TO, finds it here and takes FILE2 instead.

504EH - @FEXT: Add a Default Extension

Function 50H (request D3H, 444BH); the command loop calls it too, to add CMD (4E96H). Register Pair DE = a file specification ended by 03H, Register Pair HL = a 3 character extension. When the name has no extension, a / and the three characters are put in after the name, and the 16 bytes from the end of the name on (a password and a drive) move up 4 places. A specification that has a / is not changed. Register Pair DE comes back unchanged; Register Pair HL too when nothing is added, and otherwise one less than the address of the extension.

5066H - Put the Extension In

Register Pair HL = the byte after the name (the 03H, a period or a colon). The 16 bytes from there are moved up 4 places, then the / and the three characters of the extension are stored in the gap after the name.

507FH - Copy a Field of Letters and Digits

507FH copies a field of up to 8 characters, 5081H of up to Register B characters, from the text at Register Pair HL to Register Pair DE. Letters and digits are copied, small letters as capitals; letters and digits past the field's length are passed over. The field ends at 03H, 0DH or an open parenthesis (Register Pair HL stays at it), or at any other character (Register Pair HL passes it, and any spaces after a space). An 03H is stored after the field. Returns Register A and Register C = the character that ended the field, Register Pair DE at the 03H and the Z FLAG set when at least one character was copied (the NZ FLAG for an empty field).

50C3H - Is It a Letter or a Digit?

Returns the CARRY FLAG clear when Register A is 30H-39H (a digit) or 41H-5AH (a capital letter), and set for any other character. Register A is not changed.

50D1H - Look a Word Up in a Table

Register Pair BC = a table of 8 byte entries (a word in capitals padded with spaces to 6 characters, then two bytes) ended by 00H; Register Pair DE = the word, ended by 03H, 0DH or any character that is not a letter or a digit. Used for the command table (4E8EH), the words @FSPEC passes over (5016H) and the parameter table of @PARAM (5138H). Up to 6 characters are compared: the word matches an entry when all 6 are equal, or when the word ends where the entry has a space. Returns the Z FLAG with Register A = 00H and Register Pair DE = the entry's two bytes, or Register A = 01H and the NZ FLAG when no entry matches. Register Pair HL and Register C are kept; Register B is not.

5113H - @PARAM: Read a Parameter List

Function 60H (request E3H, 4454H) enters at 5114H. Register Pair HL = the text after a command word, Register Pair DE = the caller's parameter table: entries of 8 bytes, a parameter name padded with spaces to 6 characters and the address of the word that takes its value, ended by 00H. After any spaces the list is an open parenthesis, NAME=value or NAME entries separated by commas, and a close parenthesis. A name given alone stores FFFFH. A value is a decimal number, X'hex', a text in quotation marks (the word gets the address of the text's first character in the command line) or a word that starts with Y or ON (FFFFH), N or OF (0000H). Returns the Z FLAG when the list was read or there is none (Register A = 0DH when the line ends with no list, otherwise 00H), with Register Pair HL past the list; the NZ FLAG for a name the table does not have, an empty name, a bad value or a character that does not belong.

515DH - Read a Value

Register Pair HL = the equals sign, Register Pair BC = 0000H, Register Pair DE = the address of the parameter's word (the parameter table is on the stack). The first character of the value picks how it is read.

518EH - A Value Given as a Word: Y, N, ON, OFF

Register A = the value's first letter as a capital, Register Pair HL at it, Register Pair BC = 0000H. Y gives FFFFH, N gives 0000H, O followed by N gives FFFFH and O followed by F gives 0000H; the rest of the word (YES, NO, OFF) is passed over up to a close parenthesis, a comma or the end of the line. Any other letter returns the NZ FLAG.

51B1H - Read a Decimal Number

Register Pair HL = the first digit, Register Pair BC = 0000H. Each digit makes Register Pair BC ten times larger plus the digit (the value wraps above 65535); the first character that is not a digit ends the number, with Register Pair HL at it. The caller (517AH) stores the result whatever the flags say.

51C6H - Read a Hexadecimal Number in Quotes

Register Pair HL = the character after the X, Register Pair BC = 0000H. A quote (27H) must come first; then each hexadecimal digit (0-9, A-F, a-f) makes Register Pair BC 16 times larger plus the digit, up to a character that is not one. The closing quote is passed over when it is there. The test also takes the characters 3AH-40H as the digits 3-9 and 5BH-60H as 4-9. Returns the Z FLAG with the number in Register Pair BC, or the NZ FLAG when the quote after the X is missing.

51F0H - The Texts of SYS1/SYS (Data)

The extension the command loop gives a program name (4E93H) and the prompt it displays (4E3EH).