M-1131 MagtapeCtlPgm Nov61

This document, a "Permanent Memorandum" from Digital Equipment Corporation dated November 16, 1961, describes a "Simple Tape Control Program" (STCP) for the PDP-1 computer. Marked as "PRELIMINARY," it details a routine designed for comprehensive command and control of magnetic tape units.

Key functionalities provided by the STCP include:

• Reading: Forward and backward, in either binary or BCD format, for any specified block length.
• Writing: Forward, in binary or BCD, adhering to standard IBM format.
• Spacing: Moving forward or backward a designated number of blocks.
• Rewinding: Returning the tape transport to its load point.
• Read Check: Verifying data integrity during read operations.
• Continuous Operation: Reading or writing on a tape unit without stopping between blocks.
• Early Completion: Allowing the transport to stop at the end of a block while providing the programmer with additional milliseconds.

The program interacts through a specific calling sequence that requires six registers in core memory to define the command (e.g., operation type, direction, parity, tape unit number) and memory addresses. It can control 1 to 3 tape units and includes mechanisms for monitoring block counts and status bits. The document also outlines error detection for parity and mischaracter errors, indicating how status flags and the accumulator communicate these issues during non-normal returns. Furthermore, it discusses operational procedures, including handling inter-record gaps (with a "your gap" bit for custom timing during writing) and specific conditions for initiating tape operations.