There’s a good guide to x87 FPU by Raymond Filiatreault. Chapter 1 explains how the FPU register stack works. Yes, fld st(0) pushes a copy of the top of the stack. The Intel insn ref manual explicitly mentions this special-case use of fld st(n) I believe you’re right that fstp does pop after the fild. The first fild / fstp pair converts a global int (at L1000F140) to a … Read more
CMP subtracts the operands and sets the flags. Namely, it sets the zero flag if the difference is zero (operands are equal). TEST sets the zero flag, ZF, when the result of the AND operation is zero. If two operands are equal, their bitwise AND is zero when both are zero. TEST also sets the sign flag, SF, when the most … Read more
There is no real reason why you should. Remember that EAX is only a 32-bit register, while all the FPU registers are 80 bits in width, because the FPU does calculations on 80-bit floats by default. Therefore, moving data from the FPU register to a general purpose register will cause data loss. If you really want to … Read more
A corresponding C function would be something like Specifically, the movzbl instruction fetches the byte stored at the sum of the two parameters, zero pads it, and stores it into eax. The movsbl instruction takes the lowest byte of eax, sign extends it, and stores the result back in eax.
As others have pointed out, LEA (load effective address) is often used as a “trick” to do certain computations, but that’s not its primary purpose. The x86 instruction set was designed to support high-level languages like Pascal and C, where arrays—especially arrays of ints or small structs—are common. Consider, for example, a struct representing (x, … Read more
AT&T syntax splits the movzx Intel instruction mnemonic into different mnemonics for different source sizes (movzb vs. movzw). In Intel syntax, it’s: i.e. load a byte from memory at eax+ecx+1 and zero-extend to full register. BTW, most GNU tools now have a switch or a config option to prefer Intel syntax. (Such as objdump -Mintel or gcc -S -masm=intel, although the latter affects … Read more
x86 is the name of the architecture that it’s built to run on (the name comes from a series of old Intel processors, the names of which all ended in 86, The first of which was the 8086). Although x86 was originally a 16-bit architecture, the version in use today is the 32-bit extension. x64 … Read more
cmp arg2, arg1 performs the same operation as sub arg2, arg1 except that none of the operands are modified. The difference is not stored anywhere. However, the flags register is updated and can be used in a conditional jump, like jump-if-equal (JE), most often as the next instruction after the cmp. The advantage over other … Read more
ARM is a RISC (Reduced Instruction Set Computing) architecture while x86 is a CISC (Complex Instruction Set Computing) one. The core difference between those in this aspect is that ARM instructions operate only on registers with a few instructions for loading and saving data from / to memory while x86 can operate directly on memory as well. Up until v8 ARM was … Read more
As others have pointed out, LEA (load effective address) is often used as a “trick” to do certain computations, but that’s not its primary purpose. The x86 instruction set was designed to support high-level languages like Pascal and C, where arrays—especially arrays of ints or small structs—are common. Consider, for example, a struct representing (x, … Read more