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I found the assembly instruction
cltd
by disassembling code on an Intel architecture. The description I found was, that it clears the %edx register, but I don't understand what happens.... can anyone explain what the command exactly does?
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The topic of x86 assembly language programming is messy because: There are many different assemblers out there: MASM, NASM, gas, as86, TASM, a86, Terse, etc. All use radically different assembly languages.
There are several different assembly languages for generating x86 machine code. The one we will use in CS216 is the Microsoft Macro Assembler (MASM) assembler. MASM uses the standard Intel syntax for writing x86 assembly code.
The full x86 instruction set is large and complex (Intel's x86 instruction set manuals comprise over 2900 pages), and we do not cover it all in this guide. For example, there is a 16-bit subset of the x86 instruction set. Using the 16-bit programming model can be quite complex.
MASM uses the standard Intel syntax for writing x86 assembly code. The full x86 instruction set is large and complex (Intel's x86 instruction set manuals comprise over 2900 pages), and we do not cover it all in this guide. For example, there is a 16-bit subset of the x86 instruction set. Using the 16-bit programming model can be quite complex.
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.
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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)
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