TRS-80 DOS - NEWDOS/80 v2.0 for the Model I - SYS16/SYS Disassembled

SYS16/SYS

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SYS16/SYS implements the PDRIVE (Physical Drive) command for NEWDOS/80 v2.0. This system utility allows users to view and modify the physical drive configuration parameters stored in the Drive Configuration Table (DCT) at 4300H-439FH. Each of the 10 possible drives (0-9) has a 16-byte parameter block containing settings such as:

• TI= (Time Interval) - Drive motor-on time
• TD= (Track Density) - Not commonly used
• TC= (Track Count) - Total tracks on disk (typically 35 or 40)
• SPT= (Sectors Per Track) - Number of sectors per track (typically 10 or 18)
• TSR= (Track Stepping Rate) - Head stepping speed
• GPL= (Granules Per Lump) - Allocation unit size
• DDSL= (Directory Sectors per Lump) - Directory structure
• DDGA= (Directory Granule Allocation) - Directory space allocation

The command syntax is: PDRIVE [source=dest] [param=value...]

When invoked without parameters, PDRIVE displays the current configuration of all active drives. With parameters, it can copy drive configurations or modify individual settings. The utility validates all parameter changes and warns if incompatible settings are detected between drives (particularly the TI= specification).

Address	Bytes	Purpose
4300H-439FH	160	Drive Configuration Table - 10 drives × 16 bytes each
43A0H	1	Maximum active drive number
4EA2H	1	Current drive number being processed (0-9)
50CEH-50DBH	14	Error message: "*** ERROR ***" + 03H terminator
50DCH-5105H	42	Warning message: "**** TI= SPEC BETWEEN DRIVES INCOMPATIBLE" + 0DH
5106H-510DH	8	TI= lookup table for drive timing validation
510EH-5113H	6	PDRIVE parameter storage area
5114H	1	Source drive copy flag (nonzero = copy mode active)
5118H	1	Parse state flag
511CH+	varies	Parameter name strings: TI=, TD=, TC=, SPT=, TSR=, GPL=, DDSL=, DDGA=
4DF6H	1	Self-modifying code flag - controls "A" prefix processing

4D00H - PDRIVE Entry Point / SVC Handler:

This is the entry point for the PDRIVE command, called when the user types PDRIVE at the DOS prompt or when invoked via SVC 32H. Register A contains the SVC function code on entry. The routine first validates the function code, then parses command line parameters to either display drive configurations or modify them.

4D19H - Parse Destination Drive Number:

When PDRIVE is invoked with a parameter like "0=1" (copy drive 1 config to drive 0), this section parses the destination drive number before the equals sign.

4D3FH - Parse Parameter Name and Value:

This section handles parameter modifications like "PDRIVE 0,SPT=18" - setting specific drive parameters. It scans the parameter name table to find a match, then applies the value.

4D5BH - Parameter Error Handler:

Jump target for invalid parameter syntax errors.

4D5EH - Parse Drive Number (0-9):

Parses a single decimal digit (0-9) from the command line and returns it in Register A. Returns with CARRY set if the value is 10 or greater (invalid drive number).

4D69H - Handle Non-Numeric Parameters:

Handles parameters that take letter values (like drive letters A-P for the DDGA= parameter which specifies which drives are allocated to the directory granule area).

4D93H - Handle Single Letter Parameter:

Handles parameters that take a single letter value (A-Z) which is converted to 0-25.

4D9EH - Check For More Parameters:

After processing one parameter, check if there are more parameters on the command line. If so, loop back to parse them; otherwise proceed to update the drive configuration.

4DDAH - Display All Drive Configurations:

Called when PDRIVE is entered without parameters, or after parameters have been processed. Loops through all active drives and displays their current configuration settings.

4E16H - Display Single Drive Configuration:

Displays the configuration for a single drive. Outputs the drive number followed by all parameter names and their current values. Marks inactive drives with an asterisk.

4E5CH - Display Bitmap Parameter As Drive Letters:

Displays a 16-bit bitmap parameter as a list of drive letters (A-P). Each set bit in the bitmap corresponds to a drive letter.

4E6DH - Display Numeric Parameter:

Displays a numeric parameter value in decimal format.

4E72H - Continue Parameter Display:

Checks if there are more parameters to display, and outputs a comma separator between parameters.

4E7EH - Finish Drive Display Line:

Completes the display line for one drive. Checks for error conditions and outputs a carriage return.

4E97H - Get Parameter Type and Set IX to DCT Field:

Extracts the parameter field offset from the type byte and sets IX to point to the corresponding field in the current drive's DCT entry.

4E9AH - Calculate DCT Field Address:

Calculates the address of a specific field within a drive's DCT entry. Takes the drive number from 4EA2H and the field offset in Register A. Returns with IX pointing to that field.

4EB0H - Recalculate Derived Drive Parameters:

Recalculates the derived parameters for a drive based on its basic settings. This includes granule-to-track mappings and validation of parameter combinations. Sets error flags if invalid combinations are detected.

4F21H - Extended Parameter Validation:

Continues validation of drive parameters, checking bit flags and setting derived values. This complex section validates the relationships between multiple parameters.

4F72H - Build Drive Configuration Flags:

Builds the drive configuration flags byte based on various parameter settings. This involves checking multiple bit flags and building the result in Register E for the drive flags field.

4FC3H - Initialize Parse State:

Initializes the parsing state variables for processing PDRIVE command parameters. Sets up the parameter storage area pointer and parse state flag.

4FCCH - Check For Parameters / Handle "A" Prefix:

Checks the command line for more parameters and handles the special "A" prefix for drive specifications (allowing syntax like "PDRIVE A0,SPT=18").

4FE0H - Process After "A" Prefix Check:

Continuation of the "A" prefix handling. If "A" was found and processed, advance the pointer and continue. If not, disable further "A" prefix checking.

4FECH - Validate TI= Compatibility Across Drives:

Scans all 10 drive configurations to validate that the TI= (Time Interval) settings are compatible with each other. Returns with CARRY set if all drives are compatible, NO CARRY if incompatible.

500FH - Output Character:

Outputs a single character (in Register A) to the console via the ROM character output routine. Preserves DE and AF registers.

5017H - Search Parameter Name Table:

Searches the parameter name table (starting at BC) for a match with the text at (HL). D contains the parameter index. Returns when a match is found or end of table reached.

502EH - Parse Decimal Number Entry Point:

Main entry point to parse a decimal number from the command line. Calls the core parsing routine and validates the result.

5033H - Parse Decimal Number (Alternate Entry):

Alternate entry point that uses a different parsing routine at 505DH.

5036H - Validate Parsed Number:

Validates that the parsed number is within range (D must be zero for single-byte values).

503AH - Parameter Error Handler:

Triggers error code 2FH (47 decimal - "Parameter Error") when a parameter value is out of range.

503CH - Jump to Error Handler:

Transfers control to the SYS0 error handler with the error code in Register A.

503FH - Parse Decimal/Hex Number (Main Implementation):

Core routine to parse a decimal or hexadecimal number from the command line into Register DE. Supports hex digits A-F when preceded by decimal digits.

5057H - Validate Parse Results:

Checks that at least one digit was parsed (bit 1 of B is set by the parse loop when digits are found).

505DH - Parse Number (Alternate Entry):

Alternate entry point that pushes the validation address 5057H onto the stack, then falls into the main parse loop.

5062H - Initialize Parse State:

Initializes Register B to 00H (decimal mode) for parsing.

5064H - Initialize Accumulator and Parse Digits:

Initializes DE to 0000H and begins the digit parsing loop. Each digit is accumulated into DE.

5067H - Parse Digit Loop:

Main digit parsing loop. Reads characters, converts to digit values, and accumulates into DE. Handles both decimal (×10) and hex (×16) multiplication based on mode flag in B.

5078H - Accumulate Digit Into DE:

Multiplies DE by the base (10 for decimal, 16 for hex) and adds the new digit. Uses a clever shift-and-add algorithm.

508AH - Add Original Value for Decimal ×10:

For decimal mode, adds the original DE value to HL (currently ×4) to get ×4+×1=×5, then doubles for ×10 total.

508BH - Complete Multiplication and Add Digit:

Completes the base multiplication (×10 for decimal, ×16 for hex) and adds the new digit value to the accumulator in HL. This is reached from either decimal path (after ADD HL,DE at 508AH) or hex path (after third doubling at 5087H).

5099H - Convert 16-Bit Number to Decimal Display:

Converts a 16-bit value in DE to decimal ASCII and displays it. Uses repeated subtraction with a table of negative powers of 10 at 50C3H. Suppresses leading zeros.

509FH - Process Each Decimal Place:

Loop to process each decimal place from most significant to least.

50B7H - Display Digit:

Displays a non-zero digit (or a zero after a non-zero digit has been displayed).

50BBH - Next Decimal Place:

Decrements the digit counter and loops for remaining places, then displays the ones digit.

50C3H - Division Table (Negative Powers of 10):

Table of 16-bit negative values used for decimal conversion by repeated subtraction. Each entry is the two's complement of a power of 10.

50CBH - Padding Before Error Message:

Three space characters used as padding or prefix before the error message.

50CEH - Error Message String:

The "*** ERROR ***" message displayed when drive parameter validation fails. Terminated with 03H (ETX).

50DCH - TI= Incompatibility Warning Message:

Warning message displayed when drives have incompatible TI= (Time Interval) specifications. This occurs when different drives require different motor timing that cannot be satisfied simultaneously. Terminated with 0DH (CR).

5106H - TI= Lookup Table:

Lookup table for TI= (Time Interval) validation. Each byte represents the maximum track count for that TI= setting, used to verify drive timing compatibility. Index 0-7 corresponds to TI= values 0-7.

510EH - Parameter Storage Area:

Working storage area for parameter parsing. Contains type flags and temporary values during PDRIVE command processing.

5114H - Control Flag Storage:

Control flags used during PDRIVE command processing.

5118H - Parse State Flag:

Parse state indicator for command line processing.

511CH - Parameter Name Table:

Table of PDRIVE parameter names with their type bytes. Each entry consists of the parameter name (null-terminated) followed by a type byte that encodes: bit 7 = numeric value, bit 6 = bitmap value, low nibble = DCT field offset.

514BH - Reserved/Padding Area:

Padding area filled with NOP bytes (00H). This space may be reserved for future expansion or alignment.

516BH - Additional Parameter Names:

Additional parameters for extended PDRIVE functionality.

5178H - Set Error Flag Bit 4:

Small utility that sets bit 4 in Register E (for error flag building).

517EH - Double-Sided Disk Check:

Checks if the drive is configured for double-sided operation by testing bit 2 of the flag byte at IX+0FH. Returns error code 03H if there's a mismatch.

5186H - Reserved/Padding Area 2:

Large padding area filled with NOP bytes (00H). Reserved for future expansion.

51CBH - "A" Prefix Character Check:

Checks if the current character is "A" (allowing syntax like "PDRIVE A0,SPT=18" where "A" is an optional prefix before the drive number). This is called from 4FDDH when checking for the "A" prefix pattern.

51D3H - Check For 'A' After Decrement:

Checks if the character (after decrementing) equals 'A'. This handles the case where the original character was 'B' (42H), which becomes 'A' (41H) after DEC A.