M-1106 CoreExpan Jun61

This document details the core memory expansion options for the PDP-1 computer, offering two main systems: Type 11 and Type 14.

The Type 11 Memory System allows a PDP-1 to have up to 16,384 (16K) 18-bit words, typically using three Type 12 memory modules. It uses two 3-bit registers, Field A Register (FA) and Field B Register (FB), to address memory fields. Bit 6 of the Memory Address Register (MA) determines if FA selects addresses 0-2047 or FB selects 2048-4095, within 2048-word fields. Commands like jfd Y (jump field) are used to set the Program Counter (PC), FB, and FA, with a special defer bit to save the previous state. The cfd (change fields) command modifies FB and FA without changing the PC. High-speed channel transfers use a 14-bit address, and sequence break interrupts set FA/FB to 0 while saving their previous states.

The Type 14 Memory System provides a larger capacity of up to 32,768 (32K) words, utilizing up to seven Type 12 modules. It incorporates two 3-bit registers: the Data Field register (DF) and the Instruction Field register (IF). Each register addresses one of eight 4096-word memory modules. Instructions are typically addressed using IF and MA, while operands primarily use DF and MA. Similar to Type 11, jfd Y (jump field) sets the PC, DF, and IF, also with a defer bit option to save the prior state. A cdf (change data field) command is specific to modifying only the DF. High-speed channel transfers for Type 14 use a 15-bit address, and sequence break interrupts set DF/IF to 0, storing their previous contents.

The document recommends using the Type 11 system unless a storage capacity greater than 16,384 words is required. It concludes by presenting programming examples that illustrate memory field switching for both Type 11 and Type 14 systems, as well as for scenarios without field switching, often relying on "fetch" and "setmem" subroutines.