TRS-80 DOS - TRSDOS v2.3 for the Model I - TAPEDISK/CMD Disassembled

TAPEDISK/CMD

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TRSDOS v2.3 TAPEDISK/CMD Disassembly - Cassette to Disk (Model I)

TAPEDISK/CMD moves a machine language program from a SYSTEM cassette tape to a disk file. It shows ? and takes one of three commands: C reads a SYSTEM tape into memory at the addresses its blocks name; F filespec:d ssss eeee xxxx writes memory from ssss to eeee to the file as a load module with transfer address xxxx; E goes back to DOS READY. Anything else shows SN ERROR. Nothing is written to tape, and the program read from tape is not run.

The file is 512 bytes: two load records (5200H-52FFH, whose length byte 02H means 256 bytes, and 5300H-53F1H), a transfer record (5200H) and two bytes after it that are not part of the module. There is no name record and no comment record. The 498 loaded bytes are all on this page, each on one row, checked against S:\1\TAPEDISK.CMD and its listing.

Memory Map

The Commands

C turns the cassette motor on, disables interrupts and reads the tape (see below). The data blocks are stored wherever the tape says, with no check on the address. When the end block comes, the motor is turned off, interrupts are enabled and ? is shown again. The transfer address on the tape is read but not kept, so it has to be typed with F.

F expects the filespec after F and any spaces, with a colon and a one-character drive number, then the first address, the last address and the transfer address, each as four hexadecimal digits (capital A-F) separated by spaces, for example F PROG/CMD:1 7000 7FFF 7000. The addresses go into the operands at 5277H, 527AH and 53F2H. The FCB is the command line buffer itself, from the first character of the filespec on: 4420H opens the file or creates it, with whole 256-byte records and the sector buffer at 53F4H. The load module is built in the sector buffer by 5342H, which writes each sector as it fills: type 01H records of 256 bytes (length byte 02H) from the first address, then one record with the bytes left (length byte = the count plus 2), then the type 02H record with the transfer address. The part-filled last sector is written, the file is closed and ? is shown again. A disk error shows its message through 4409H (with bit 7 set, so it comes back) and returns to ?; the file is not closed then.

E jumps to 402DH, DOS READY.

The Tape Format Read

Each bit is a clock pulse followed by a data pulse for 1 or no pulse for 0. A pulse sets bit 7 of port FFH and any output to port FFH clears it. 53B0H waits for bit 7 (the clock pulse), waits 65 loop turns, clears the latch, waits 118 turns and reads bit 7 as the data bit, then clears the latch again. The search for the sync byte A5H goes bit by bit; after it, the bytes read are:

Notes from the Code

• A bad checksum only puts a C at 3C3EH: the block stays in memory as read and the next block is read. The C stays on the screen.
• BREAK is not looked at while the tape is read; a tape that never gives the 78H block leaves TAPEDISK reading until RESET.
• A tape block can be stored over TAPEDISK itself (5200H-53F3H) or over TRSDOS, as the addresses are not checked.
• 5232H looks for the colon without a limit: a filespec without a drive number makes it search past the line typed.
• The count of bytes F writes is split by DEC H and a test of the SIGN FLAG (5282H): from 8100H bytes on, no 256-byte records are written, only the record of the bytes left in Register L.
• The part of the last sector after the module holds what the buffer held before; the loader stops at the 02H record, so it is never read.
• Five NOP bytes follow the C test (521DH-5221H), and a second JP 5200H at 5324H is not reached.
• 53E7H tests the motor bit, but no caller uses the result.

Variables and Changed Operands

5200H - TAPEDISK Starts: the Prompt and the Command

SYS1/SYS loads TAPEDISK/CMD at 5200H and jumps here; every command comes back here too. Shows ? on a new line and reads a line of up to 63 characters into the command line buffer at 4318H. The first character picks the command: C reads a SYSTEM tape into memory (52CFH), E goes back to DOS READY (402DH), F writes memory to a disk file (522CH); anything else shows SN ERROR.

522CH - The F Command: Read the File Name and the Addresses

The line is F filespec:d ssss eeee xxxx, with ssss the first address, eeee the last address and xxxx the transfer address, each exactly four hexadecimal digits (0-9, A-F). The filespec is not copied: its place in the command line buffer serves as the FCB, and its address is stored at 5269H. The four digits of each address are turned into bytes and stored into the operands of the code that uses them: 5277H-5278H, 527AH-527BH and 53F2H-53F3H. A character that is not a hexadecimal digit gives SN ERROR (53A0H).

5268H - The F Command: Open the File and Write the Load Module

Opens or creates the file with whole 256-byte records and the sector buffer at 53F4H, then writes a load module through 5342H, which collects the bytes at 53F4H and writes each full sector: type 01H records of 256 bytes from the first address on, one shorter record for the rest, and a type 02H record with the transfer address. The last part of a sector is written, the file is closed, and ? is shown again. A disk error shows its message (4409H) and goes back to the prompt.

52CFH - The C Command: Read a SYSTEM Tape into Memory

Starts the cassette motor and reads bits until the sync byte A5H, then waits for the header byte 55H and skips the six bytes of the name. Then it reads blocks: 3CH, a count (00H = 256), a load address and the data, which is stored at that address, then a checksum. After a good block the character at the top right of the screen (3C3FH) is changed (XOR 0AH turns an asterisk into a space and back); after a bad one a C is put at 3C3EH and reading goes on. The block 78H ends the tape: its address is read but not used, the motor is stopped and ? is shown again. Interrupts are off while the tape is read; BREAK is not looked at.

5327H - Put a Load Record into the File

Register B = the number of data bytes (00H = 256), Register Pair DE = the address of the first one. Puts the record type 01H, the length (Register B plus 2, for the two address bytes), the address low byte and high byte, and the data bytes through 5342H. Returns Register Pair DE past the last byte and Register B = 00H.

5342H - Put a Byte into the Sector Buffer

Register A = the byte, Register C = its position in the buffer at 53F4H. Stores the byte at 53F4H plus Register C and counts Register C up; when it wraps to 00H the 256 bytes are full and are written as the next sector of the file (4439H). A write error goes to 52BCH. Register Pair DE and Register B are kept.

535EH - Skip to the Next Word

Register Pair HL = the text. Moves Register Pair HL on by one character, then past any spaces. Returns Register Pair HL = the first character that is not a space and Register A = that character.

5365H - Read an Address from the Tape

Reads two bytes through 53A6H, low byte first, into Register Pair HL. Returns Register A = the high byte.

536EH - Display a Text

Register Pair HL = the text, Register B = its length. Shows Register B characters through the ROM.

5376H - Two Hexadecimal Digits to a Byte

Register Pair HL = the first of two hexadecimal digits. Returns Register A = their value, with Register Pair HL on the second digit. A character that is not 0-9 or A-F makes 538AH return to 52C4H (SN ERROR) instead.

538AH - One Hexadecimal Digit

Register A = the character. Returns Register A = 00H-09H for 0-9 and 0AH-0FH for A-F (capital letters only). Any other character goes to 53A0H, which makes 5376H return to 52C4H.

53A6H - Read a Byte from the Tape

Reads eight bits through 53B0H, the first bit read ending up in bit 7. Returns Register A = the byte; Register Pair BC is kept.

53B0H - Read One Bit from the Tape

Register A = the bits read so far. Each bit on the tape is a clock pulse, then a pulse for a 1 or none for a 0. Bit 7 of port FFH is set by a pulse and cleared by any output to port FFH. The routine waits for the clock pulse, waits 41H loop turns, clears the latch, waits 76H loop turns more and reads the latch: bit 7 is the bit. The latch is cleared again for the next clock pulse. Returns Register A shifted left with the new bit in bit 0; the main Register Pairs BC, DE and HL are kept by using the alternate set.

53D0H - Cassette Motor and Port FFH

Each entry loads Register Pair HL with a mask in Register H and bits to set in Register L for the ROM's copy of port FFH at 403DH: 53D0H clears bit 2 (motor off), 53D5H sets bit 2 (motor on), 53DAH changes nothing. 53DDH sends the result to port FFH, which also clears the cassette pulse latch, and stores it at 403DH.

53EAH - The Message and the Data Areas

The one message and, past the end of the file, the transfer address of the F command and the sector buffer of the file being written.