Eleanor muttered, “Oh, you ancient beast.”
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.
That night, Eleanor poured a whiskey and thought about LDS . Born in 1978 with the 8086, mature in the 286’s protected mode, and already a zombie on the 386—kept alive only by backward compatibility. It was the programming equivalent of a rotary phone in a smartphone world. You could still use it. But you really, really shouldn’t.
She couldn’t just remove the LDS . The entire linked list traversal depended on far pointers. But she could replace it. x86 lds
The offending line looked innocent:
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.
And somewhere in a museum, a 386 motherboard smiled, its LDS instruction still perfectly capable of crashing any program that dared to wake it. Eleanor muttered, “Oh, you ancient beast
After patching, the model ran. It plotted Devonian shale layers for three hours without a single fault.
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 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. But it was a time bomb
In the spring of 1992, Eleanor, a young and slightly reckless systems programmer, found herself hunched over a beige 386 DX/40. The machine groaned under MS-DOS 5.0, and in front of her was a nightmare: a core dump from a geological modeling program she’d inherited.
A decade later, she’d tell interns: “ LDS loads a pointer and destroys your data segment. Respect it. Then avoid it.”
The disassembly pointed to one instruction: LDS .
The GPF happened when LDS tried to read from DS:SI —but DS had been clobbered by an interrupt handler. So LDS cheerfully loaded garbage into DS itself, because that’s what LDS does: it writes the segment part of the loaded pointer directly into the DS register. Now DS pointed to an unmapped address. The next instruction—a simple mov ax, [bx] —caused the system to keel over.