TRS-80 DOS - TRSDOS v2.3 for the Model I - SYS1/SYS Disassembled

SYS1/SYS

Other Navigation

TRSDOS v2.3 SYS1/SYS Disassembly - The Command Interpreter (Model I)

SYS1/SYS is the TRSDOS 2.3 system overlay that shows DOS READY, reads a command line and decides what to do with it. SYS0/SYS loads it into the overlay area at 4E00H whenever an RST 28H code with 3 in bits 0-3 is executed, and enters it at 4E00H with that code in Register A. Besides the prompt it gives the other modules three services: parsing a filespec into an FCB, adding a default extension, and parsing a parameter list such as (ON) or (NAME=X'4000').

Three commands are carried out inside SYS1/SYS itself: BASIC2, DEBUG and TRACE. The nineteen library commands (APPEND to VERIFY) are handed to SYS6/SYS with their number in Register C. Any other name is loaded and run as a program, with /CMD added when the name has no extension.

The file holds 963 bytes in five load records covering 4E00H-51AEH and 51B5H-51C8H, after a name record and a 169-byte copyright comment record, and its transfer address is 4E00H. Every byte is shown once on this page, in address order, checked against S:\1\SYS1.SYS and its listing. The six bytes 51AFH-51B4H between the records are not in the file and are shown as a DEFS row.

Memory Map

Functions

SYS0/SYS's fixed entry points load Register A with an RST 28H code and execute RST 28H. Bits 0-3 of every code below are 3 (SYS1/SYS); bits 4-6 are the function that 4E00H tests.

Commands

The command word is parsed as a filespec into SYS0/SYS's command FCB at 4480H and looked up first in the table at 4EA4H, then in the table at 4EBDH. Each table entry is eight bytes: the name padded with spaces to six characters, then a word. Only the first six characters of the typed name are compared.

Parameter Lists

A parameter list follows the command in parentheses: (NAME=value,NAME,...). The caller gives a table in the same eight-byte layout, whose word is the address of a two-byte variable. A name with no equals sign stores FFFFH. A value can be X'hex digits', a decimal number, or a word starting with Y, N, ON or OF, which stores FFFFH (YES, ON) or 0000H (NO, OFF). DEBUG and TRACE read only a single (ON) or (OFF) through 51A0H.

Notes from the Code

• The lookup at 502AH compares six characters and does not check that the name ends there, so APPENDXY runs APPEND. A name that ends in a slash, period or colon right after two letters also matches a two-letter entry, so a filespec such as TO/CMD is taken as the noise word TO.
• A six-character parameter name ends with 03H at 51B5H (5095H), over the C of the default extension CMD. SYS1/SYS is read from the diskette again on its next RST 28H call unless it is still in memory, so the damage lasts only while it stays loaded.
• The YES/NO reader at 50DFH passes the rest of the word until a comma or closing parenthesis and does not stop at ENTER; (ON followed by ENTER makes it search the memory after the command line.
• The hexadecimal reader at 511FH takes the characters colon to at-sign as the digits 3 to 9, and ends with the Z FLAG at any other character even when the closing quote is missing.
• DEBUG or TRACE followed by a parenthesis and any word other than Y or ON turns the feature off (51A0H).
• The BIT tests of the prompt at 4E1FH read the ROM byte whose address is the word at 430FH-4310H, not the system flags themselves.

Variables in SYS1/SYS

Addresses Outside SYS1/SYS

4E00H - Function Dispatcher

SYS0/SYS's overlay call (4BFBH) enters SYS1/SYS here with Register A = the RST 28H code whose bits 0-3 are 3. Bits 4-6 select the function: 10H (code 93H, DOS READY from 4400H and 402DH) and 20H (code A3H, the exit after an error at 4030H) both go to the command prompt, 30H (B3H, 4405H) runs the command at Register Pair HL, 40H (C3H, 441CH) parses a filespec, 50H (D3H, 4473H) adds a default extension and 60H (E3H, 4476H) parses a parameter list. Register Pair BC, Register Pair DE and Register Pair HL are the caller's.

4E1FH - DOS READY: the Command Prompt

Entered for functions 10H and 20H and from the DEBUG command (5181H). Interrupts are enabled and the stack is reset to 41FCH. The BIT tests at 4E26H-4E34H work on the byte whose address is the word at 430FH-4310H (the system flags and the spare byte 4310H, 00H): with 430FH = 01H they read the ROM's byte at 0001H (AFH), which arms the BREAK vector at 4315H and makes a held BREAK key exit through 4030H; with 11H (SYS6/SYS in memory) the byte at 0011H (03H) does neither. Then DOS READY is displayed and a command line of up to 63 characters is read into 4318H.

4E4EH - Run a Command Line

Entered from 4E4BH and for function 30H (4405H: the AUTO command, BASIC/CMD). Register Pair HL = the command text. The first word is parsed as a filespec into the command FCB at 4480H. A name in the first table (BASIC2, DEBUG, TRACE) runs its handler here; a name in the second table (the library commands) runs SYS6/SYS with the command's number in Register C; any other name is run as a program with the default extension /CMD. Every command returns to 402DH (DOS READY).

4E90H - Run a Library Command (SYS6/SYS)

The handler of every entry of the second table. Register C = the command number (1 APPEND ... 19 VERIFY). When bit 4 of the system flags at 430FH shows SYS6/SYS is already at 5200H it is entered directly; otherwise the bit is set and SYS6/SYS is loaded through RST 28H code 88H (overlay 8), whose transfer address 5200H dispatches on Register C.

4EA4H - First Command Table

Eight bytes per entry: the command name padded with spaces to six characters, then the handler address. A 00H byte ends the table. 502AH returns the handler in Register Pair DE and the entry number in Register C.

4EBDH - Second Command Table: the Library Commands

Nineteen entries of eight bytes, all with the handler 4E90H, so the entry number (Register C, 1-19, counted by 502AH) tells SYS6/SYS which command to run. A 00H byte ends the table.

4F56H - Parse a File Specification (441CH)

Register Pair HL = the text, Register Pair DE = the FCB. Copies NAME/EXT.PASSWORD:D into the FCB as text: up to 8 letters or digits for the name, then optionally a slash and up to 3 for the extension, a period and up to 8 for the password, a colon and 1 for the drive, in that order, and puts 03H after the last field. A name that is empty but starts with an asterisk is a device name: the asterisk and up to 2 characters. When the name is TO, ON or OVER (table 4FA7H) the next word is parsed into the same FCB instead. Returns Register A = 00H and the Z FLAG with Register Pair HL past the delimiter that ended the filespec, or the NZ FLAG for an empty field.

4FA7H - Noise Word Table

Words that 4F56H skips, in the same eight-byte layout as the command tables (six characters, then a word that is not used here). A 00H byte ends the table. They let a command read COPY A TO B or RENAME A TO B.

4FC0H - Add a Default Extension (4473H)

Register Pair DE = an FCB holding a filespec as text (from 4F56H), Register Pair HL = a three-character extension (51B5H, CMD, for programs). When the name already has a slash, nothing is done. Otherwise the slash and the extension are inserted where the name ends (at a period, colon or 03H), moving the rest of the filespec up four bytes.

4FF4H - Copy One Field of a Filespec

Register Pair HL = the text, Register Pair DE = where the field goes, Register B = the most characters to keep. Copies letters and digits, skipping (not storing) any beyond the limit, until a character that is neither: 03H and 0DH end the field without being passed; anything else is passed over and returned as the delimiter. 03H is stored after the field. Returns Register A = the delimiter and the Z FLAG when at least one character was copied (the NZ FLAG for an empty field).

502AH - Look Up a Name in a Table

Register Pair DE = a name as text ending in 03H or 0DH (an FCB at 4480H or the buffer at 51AFH), Register Pair BC = a table of eight-byte entries (six characters padded with spaces, then a word) ending with a 00H byte. Returns the Z FLAG on a match with Register Pair DE = the entry's word, Register C = the entry number counting from 1 and Register B = the high byte of the entry's address; returns the NZ FLAG with Register Pair DE unchanged when no entry matches. Register Pair HL is preserved. Only the first six characters of the name are compared, so a longer name such as APPENDXY matches APPEND.

507DH - Parse a Parameter List (4476H)

Register Pair HL = the command line after the command, Register Pair DE = a parameter table in the eight-byte layout of 502AH, whose word is the address of a two-byte variable. Reads (NAME=value,NAME,...): each name is looked up and its variable gets the value, FFFFH when there is no equals sign. A value is X followed by hexadecimal digits in quotes, a decimal number, or a word starting with Y, N, ON or OF (YES/NO/ON/OFF give FFFFH or 0000H). Returns the Z FLAG with Register Pair HL past the closing parenthesis, or at the ENTER when there is no list; returns the NZ FLAG for an unknown name or bad text.

50DFH - Read YES, NO, ON or OFF

Register Pair HL = the word. Only the first letter is tested for Y and N and the first two for ON and OF. Returns the Z FLAG with Register Pair BC = FFFFH for Y or ON, 0000H for N or OF, and Register Pair HL at the comma or closing parenthesis that follows the word; returns the NZ FLAG with Register Pair BC = 0000H for any other word. The skip loop at 50FAH stops only at a comma or a closing parenthesis, so a list without one is searched past its ENTER into the memory that follows.

5104H - Read a Decimal Number

Register Pair HL = the first digit. Returns Register Pair BC = the value of the digits that follow, with Register Pair HL at the first character that is not a digit. A value above 65535 keeps only its low sixteen bits; there is no error return.

511FH - Read a Quoted Hexadecimal Number

Register Pair HL = the character after the X. Reads 'digits' with the digits 0-9 and A-F. Returns the Z FLAG with Register Pair BC = the value and Register Pair HL past the closing quote, or the NZ FLAG when no opening quote is there. Any other character also ends the number with the Z FLAG, Register Pair HL at that character; the characters colon to at-sign (3AH-40H) are taken as the digits 3 to 9.

514CH - BASIC2 Command

Entry 1 of the table at 4EA4H. Leaves TRSDOS 2.3 for the ROM's Level II BASIC: copies the ROM's initial values for 4000H-4035H back over the RST vectors and the device control blocks, clears 4036H-405CH and enters the ROM's start-up code at 0075H, which never returns to TRSDOS 2.3.

5162H - DEBUG Command

Entry 2 of the table at 4EA4H. DEBUG and DEBUG (ON) or (YES) set bit 7 of the system flags at 430FH, turn the BREAK vector at 4315H into JP 400FH and the exit after an error at 4030H into CALL 400FH, so both the BREAK key and the next error enter DEBUG through the RST 30H vector. DEBUG (OFF) or (NO), and any other word in parentheses, clear bit 7 and 4315H and put back LD A,A3H / RST 28H at 4030H.

5191H - TRACE Command

Entry 3 of the table at 4EA4H. TRACE or TRACE (ON) puts the task at 4CD9H (SYS0/SYS), which shows the interrupted program counter on the screen, into timer slot 11; TRACE (OFF) removes it.

519FH - Read ON or OFF

Register Pair HL = the command line after DEBUG or TRACE. Skips spaces; with no opening parenthesis returns the NZ FLAG (ON). With one, reads the word through 50DFH and returns the NZ FLAG for Y or ON and the Z FLAG for N, OF or any other word.

51AFH - Data

The parameter name buffer is not in the file. The default extension and the two messages follow it in the last load record (51B5H-51C8H).