trying to understand the assembly instruction: cltd on x86

It looks pretty straightforward to me: cltd converts the signed long in EAX to a signed double long in EDX:EAX by extending the most-significant bit (sign bit) of EAX into all bits of EDX."

cltd converts signed long to signed double long

If you want to see a diagram of what happens, jump to page 160 of http://download.intel.com/products/processor/manual/325462.pdf (more details on page 681)

cltd is an alias for cdq (reference), which sign-extends eax into edx:eax.

What this means in practice is that edx is filled with the most significant bit of eax (the sign bit). For example, if eax is 0x7F000000 edx would become 0x00000000 after cdq. And if eax is 0x80000000 edx would become 0xFFFFFFFF.