1.19. FLOATING-POINT UNIT
92 es 0x7b 123
93 fs 0x0 0
94 gs 0x33 51
95 (gdb) ni
96 0x080484ae in d_max ()
97 (gdb) info float
98 R7: Valid 0x4000d999999999999800 +3.399999999999999911
99 =>R6: Valid 0x4000d999999999999800 +3.399999999999999911
100 R5: Empty 0x00000000000000000000
101 R4: Empty 0x00000000000000000000
102 R3: Empty 0x00000000000000000000
103 R2: Empty 0x00000000000000000000
104 R1: Empty 0x00000000000000000000
105 R0: Empty 0x00000000000000000000
106
107 Status Word: 0x3000
108 TOP: 6
109 Control Word: 0x037f IM DM ZM OM UM PM
110 PC: Extended Precision (64-bits)
111 RC: Round to nearest
112 Tag Word: 0x0fff
113 Instruction Pointer: 0x73:0x080484ac
114 Operand Pointer: 0x7b:0xbffff118
115 Opcode: 0x0000
116 (gdb) disas $eip
117 Dump of assembler code for function d_max:
118 0x080484a0 <+0>: fldl 0x4(%esp)
119 0x080484a4 <+4>: fldl 0xc(%esp)
120 0x080484a8 <+8>: fxch %st(1)
121 0x080484aa <+10>: fucomi %st(1),%st
122 0x080484ac <+12>: fcmovbe %st(1),%st
123 => 0x080484ae <+14>: fstp %st(1)
124 0x080484b0 <+16>: ret
125 End of assembler dump.
126 (gdb) ni
127 0x080484b0 in d_max ()
128 (gdb) info float
129 =>R7: Valid 0x4000d999999999999800 +3.399999999999999911
130 R6: Empty 0x4000d999999999999800
131 R5: Empty 0x00000000000000000000
132 R4: Empty 0x00000000000000000000
133 R3: Empty 0x00000000000000000000
134 R2: Empty 0x00000000000000000000
135 R1: Empty 0x00000000000000000000
136 R0: Empty 0x00000000000000000000
137
138 Status Word: 0x3800
139 TOP: 7
140 Control Word: 0x037f IM DM ZM OM UM PM
141 PC: Extended Precision (64-bits)
142 RC: Round to nearest
143 Tag Word: 0x3fff
144 Instruction Pointer: 0x73:0x080484ae
145 Operand Pointer: 0x7b:0xbffff118
146 Opcode: 0x0000
147 (gdb) quit
148 A debugging session is active.
149
150 Inferior 1 [process 30194] will be killed.
151
152 Quit anyway? (y or n) y
153 dennis@ubuntuvm:~/polygon$
Using “ni”, let’s execute the first twoFLDinstructions.Let’s examine the FPU registers (line 33).As it was mentioned before, the FPU registers set is a circular buffer rather than a stack (1.19.5 on
page 225). And GDB doesn’t showSTxregisters, but internal the FPU registers (Rx). The arrow (at line
35) points to the current top of the stack.