9.16.9 Floating Point

All 80387 floating point types except packed BCD are supported. (BCD support may be added without much difficulty). These data types are 16-, 32-, and 64- bit integers, and single (32-bit), double (64-bit), and extended (80-bit) precision floating point. Each supported type has an instruction mnemonic suffix and a constructor associated with it. Instruction mnemonic suffixes specify the operand’s data type. Constructors build these data types into memory.

• Floating point constructors are ‘.float’ or ‘.single’, ‘.double’, and ‘.tfloat’ for 32-, 64-, and 80-bit formats. These correspond to instruction mnemonic suffixes ‘s’, ‘l’, and ‘t’. ‘t’ stands for 80-bit (ten byte) real. The 80387 only supports this format via the ‘fldt’ (load 80-bit real to stack top) and ‘fstpt’ (store 80-bit real and pop stack) instructions.
• Integer constructors are ‘.word’, ‘.long’ or ‘.int’, and ‘.quad’ for the 16-, 32-, and 64-bit integer formats. The corresponding instruction mnemonic suffixes are ‘s’ (single), ‘l’ (long), and ‘q’ (quad). As with the 80-bit real format, the 64-bit ‘q’ format is only present in the ‘fildq’ (load quad integer to stack top) and ‘fistpq’ (store quad integer and pop stack) instructions.

Register to register operations should not use instruction mnemonic suffixes. ‘fstl %st, %st(1)’ will give a warning, and be assembled as if you wrote ‘fst %st, %st(1)’, since all register to register operations use 80-bit floating point operands. (Contrast this with ‘fstl %st, mem’, which converts ‘%st’ from 80-bit to 64-bit floating point format, then stores the result in the 4 byte location ‘mem’)