F.F. Loader

This document details the F.F. Loader, a program designed for the PDP-7 computer to load binary object tapes produced by the PDP-7 Assembly System (Digital-7-3-5).

The F.F. Loader operates within memory locations 17600-17755 and is typically initiated using the Read-In Mode Loader (RIM). Its core functions include:

• Checksum Verification: During read-in, it computes and compares a checksum against the value read from the tape, halting and displaying errors in the Accumulator (AC) if a mismatch occurs.
• Program Execution: After successful loading, the loader executes instructions specified in the object tape's start block. The AC will then hold the address following the last loaded constant, usually the first free memory location.
• F.F. Binary Format Processing: The loader interprets data in a unique F.F. Binary format. Each data entry consists of three lines of tape, providing 24 bits of information (18 data bits and 6 bits of loader instructions). Data words are loaded into memory sequentially downwards from a highest specified location, managed by a Current Address Indicator (CAI).
• Loader Handling Codes: A set of 4-bit codes dictates how the loader processes each data word, covering actions such as direct storage, resolving undefined symbols, defining symbols, and managing constants by searching and adding to a constants table.
• System Integration: It is compatible with 4K and 8K PDP-4/7 machines and uses specific memory locations (7 and 10) for temporary storage during the loading process.
• Control Flow: The loader handles various program termination directives (START or PAUSE, with or without an address), either transferring control to a specified address or halting the machine.

The document further elaborates on the structure of F.F. Binary words, the specific meanings of each loader handling code, and operational characteristics concerning checksum error handling and the management of undefined symbols during assembly and loading.