The x86 processors have eight 32-bit general-purpose registers. The names of some of these general-purpose registers are EAX, EBX, ECX, and EDX. These registers can be used in subsections. For example, the least significant 2 bytes of EAX can be used as a 16-bit register called AX. Again, the least significant byte of AX can be used as an 8-bit register called AL and the most significant byte of AX can be used as an 8-bit register called AH. Similarly, the BX register can be used in the form of the BH and BL registers, and so on.
We will be writing inline assembly code in this chapter because this code is easily integrated with C code during code generation. Consequently, the C and assembly code is optimized by the compiler to produce efficient object code.
The syntax for using inline assembly code is as follows:
asm [volatile] (
asm statements
: output statements
: input statements
);
The asm statements are enclosed within quotes, and the outputs and inputs are in the form of "constraint" (name) pairs separated by commas. The constraints can be any of the following:
| Constraint | Usage |
| g | Compiler will decide the register to be used for the variable |
| r | Load into any available register |
| a | Load into the eax register |
| b | Load into the ebx register |
| c | Load into the ecx register |
| d | Load into the edx register |
| f | Load into the floating-point register |
| D | Load into the edi register |
| S | Load into the esi register |