While 896K provided a massive boost for memory-resident programs (TSRs) and large spreadsheets, it introduced several risks:
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
Breaking the 640K Barrier: The Legacy of 896K.txt and Early PC Memory Expansion 896K.txt
Achieving 896K required more aggressive "filling" of the upper memory area (UMA). This involved:
). On systems using standard MDA or CGA video cards, this segment was often empty. By installing RAM chips that responded to these addresses, users could gain an additional 64K, bringing the total contiguous memory to . Standard MS-DOS/PC-DOS versions could recognize this memory without a specialized BIOS. 2.2 Reaching 896K While 896K provided a massive boost for memory-resident
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
Because the IBM BIOS did not officially support RAM in these segments, some software that bypassed DOS to check memory limits would fail to "see" the extra space. 4. Historical Significance On systems using standard MDA or CGA video
were "reserved," they were not always fully occupied by hardware. 2.1 The A Segment (704K Expansion) The most common expansion involved the A segment ( A0000cap A 0000 AFFFFcap A cap F cap F cap F cap F