mov %eax, %ebx between general-purpose registers is one of the most common instructions. Modern hardware supports it extremely efficiently, often with special cases that don’t apply to any other instruction. On older hardware, it’s always been one of the cheapest instructions. On Ivybridge and later, it doesn’t even need an execution unit and has zero … Read more
The segment limit in real mode is 64K, even on a 386 or later CPU where you can use 32-bit address-size via prefixes. e.g. mov ax, [edx + ecx*4] is still limited to offsets of 64 KiB in real mode. If you exceed this limit, it raises a #GP exception on 286+. (Or #SS if … Read more
Some historical background The 8086 always used a fixed 64KiB Window per segment whose starting address was calculated by (segment register * 16). Since the 80286 there are some special tables in memory (GDT and LDT). These tables contain the starting address, the length and the access rights of a segment. The segment registers (CS, … Read more
On implementations with a flat memory model (basically everything), casting to uintptr_t will Just Work. (But see Should pointer comparisons be signed or unsigned in 64-bit x86? for discussion of whether you should treat pointers as signed or not, including issues of forming pointers outside of objects which is UB in C.) But systems with … Read more
You would be required to have either the base or the offset to start with, along with the linear address, as multiple Segment:Offset pairs can map to the same linear address. so if we have the segment 0xF400 and the linear address 0xF4170, we get the offset being 0xF4170 – (0xF400 << 4) which is … Read more