HP gnu source-level debugger 5992-4701 Printing Floating Point Values, Floating point hardware

Some registers have distinct raw and virtual data formats. This means that the data format in which the register contents are saved by the operating system is not the same one that your program normally sees. For example, the registers of the 68881 floating point coprocessor are always saved in “extended” (raw) format, but all C programs expect to work with “double” (virtual) format. In such cases, GDB normally works with the virtual format only (the format that makes sense for your program), but the info registers command prints the data in both formats.

Normally, register values are relative to the selected stack frame (see “Selecting a frame” (page 73)). This means that you get the value that the register would contain if all stack frames farther in were exited and their saved registers restored. In order to see the true contents of hardware registers, you must select the innermost frame (with 'frame 0').

However, GDB must deduce where registers are saved, from the machine code generated by your compiler. If some registers are not saved, or if GDB is unable to locate the saved registers, the selected stack frame makes no difference.

8.11 Printing Floating Point Values

You can print the values of floating-point registers in different formats.

To print both single and double-precision values:

(gdb) info reg $fr5

fr5 (single precision) 10.1444092 fr5

To get the bit pattern, try the following macro:

define pbits

set *((float *) $sp)=$arg0 p/x *((int *) $sp)

end

This is what the macro produces:

(gdb) pbits $fr6 $1 = 0x4082852d

8.12 Floating point hardware

Depending on the configuration, GDB may be able to give you more information about the status of the floating point hardware.

info float Display hardware-dependent information about the floating point unit. The exact contents and layout vary depending on the floating point chip. Currently, 'info float' is supported on the ARM and x86 machines.

8.11 Printing Floating Point Values 99