X86 Lds [ Cross-Platform ]

She couldn’t just remove the LDS . The entire linked list traversal depended on far pointers. But she could replace it.

Eleanor muttered, “Oh, you ancient beast.”

“It poisoned its own segment register,” Eleanor whispered. “Like a snake biting its tail.” x86 lds

The code was a fossil, written in a hybrid of C and inline assembly by a geophysicist who had long since retired to a cabin without electricity. The error was a General Protection Fault (GPF)—the 386’s way of screaming, “You touched memory you don’t own.”

She knew LDS —Load Pointer Using DS. A relic from the segmented memory model of the 16-bit era, when pointers were 32-bit monsters: a 16-bit segment and a 16-bit offset. On her 32-bit 386, it still worked—mostly. But it was a time bomb. She couldn’t just remove the LDS

lds bx, [si] ; Load 32-bit pointer from address DS:SI into DS:BX The geophysicist had used it to chase a linked list of fault lines. Eleanor realized the bug: the code assumed SI pointed to a far pointer stored in the current data segment. But in protected mode, under a DOS extender, DS could change anytime a task switched. One moment DS pointed to low memory; the next, to a buffer in extended memory.

She wrote a small C helper using memcpy to safely read the 32-bit value into a local unsigned long , then manually set DS and BX via __asm —but with interrupts disabled via _disable() . Clunky, but safe. Eleanor muttered, “Oh, you ancient beast

The offending line looked innocent:

A decade later, she’d tell interns: “ LDS loads a pointer and destroys your data segment. Respect it. Then avoid it.”