Breaking the 640K Barrier: The Legacy of 896K.txt and Early PC Memory Expansion
The original IBM PC architecture utilized the Intel 8088 processor, which had a 20-bit address bus capable of addressing 1 MB ( 2202 to the 20th power 896K.txt
Achieving 896K required more aggressive "filling" of the upper memory area (UMA). This involved: Breaking the 640K Barrier: The Legacy of 896K
bytes) of memory. IBM reserved the upper 384 KB for hardware-specific tasks (BIOS, video memory, and expansion cards), leaving 640 KB for the operating system and applications. As software grew in complexity, enthusiasts sought ways to reclaim reserved address space for system RAM. 2. Technical Methodology As software grew in complexity, enthusiasts sought ways
On the IBM XT (5160) motherboard, this often required altering the decoder logic (e.g., using a 74LS138 decoder) to enable the RAM parity checks and chip select signals for these high addresses. 3. Compatibility and Limitations