### Eyecatchers

An eyecatcher is a recognizable sequence of bytes that tells unprototyped code which parameters are passed in which registers. Eyecatchers apply only to code without prototype statements.

The eyecatcher instruction is placed after the call instruction for a nonprototyped function. The choice of instruction for the eyecatcher relies on the fact that the TEST instruction does not modify the referenced register, meaning that the return register of the call instruction is not modified by the eyecatcher instruction. The absence of an eyecatcher in unprototyped code implies that there are no parameters in registers. (Note that this eyecatcher scheme does not allow the use of execute-only code segments.)

The eyecatcher has the format:

       TEST EAX, immed32

Note that the short-form binary encoding (A9) of `TEST EAX` must be used for the eyecatcher instruction.

The 32-bit immediate operand is interpreted as a succession of 2-bit fields, each of which describes a register parameter or a 4-byte slot of stack memory. Because only one 32-bit read of the eyecatcher is made, only 24 bits of the immediate operand are loaded. The actual number of parameters that can be considered for registers is restricted to 12.

Because of byte reversal, the bits that are loaded are the low-order 24 bits of the 32-bit immediate operand. The highest-order 2-bit field of the 24 bits analyzed corresponds to the lexically first parameter, while subsequent parameters correspond to the subsequent lower-order 2-bit fields. The meaning of the values of the fields is as follows:

Value
  Meaning
00
  This value indicates that there are no parameters remaining to be put into registers, or that there are parameters that could be put into registers but there are no registers remaining. It also indicates the end of the eyecatcher.
01
  The corresponding parameter is in a general-purpose register. The leftmost field of this value has its parameter in EAX, the second leftmost (if it exists) in EDX, and the third (if it exists) in ECX.
10
  The corresponding parameter is in a floating-point register and has 8 bytes of stack reserved for it (that is, it is a double). ST(0), ST(1), ST(2), and ST(3) contain the lexically-first four floating-point parameters (fewer registers are used if there are fewer floating-point parameters). ST(0) contains the lexically first (leftmost 2-bit field of type 10 or 11) parameter, ST(1) the lexically second parameter, and so on.
11
  The corresponding parameter is in a floating-point register and has 16 bytes of stack reserved for it (that is, it is a long double.)

Calling Conventions