Memory Direct Access by Old Computers

Photo by Possessed Photography on Unsplash

Early computers like the Apple II allowed programmers to manipulate memory directly due to their simple architecture and lack of modern operating system protections, which made the hardware fully accessible to software. Let’s break this down by looking at how it worked back then, why it’s different now, and the reasons behind these changes.

How Early Computers Like the Apple II Allowed Direct Memory Manipulation

1. Simple Hardware Architecture: The Apple II, released in 1977, was built around the MOS 6502 microprocessor, an 8-bit CPU with a straightforward design. It had a 16-bit address space, allowing access to 64KB of memory, and there were no built-in mechanisms to restrict access to specific memory regions. Programmers could read from or write to any memory address directly using assembly language instructions like LDA (load accumulator) or STA (store accumulator). For example, writing to address $C000 could toggle a hardware switch or update the screen display because memory was directly mapped to hardware functions.
2. No Operating System Barriers: The Apple II typically booted into a BASIC interpreter (like Applesoft BASIC) or ran programs directly from disk or cassette without a sophisticated operating system. There was no concept of user permissions, kernel space, or memory…