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Coding Examples
For example, assume a stack frame as described in Figure 2-25, with the back chain word
at address 0x400, and where the CR does not need to be saved. To save only 32-bit values,
r11 should contain 0x400 on entry to the 32-bit register save routine. To save only 64-bit
values, r11 should contain 0x400 - 0x90 = 0x370. Figure 2-32 shows an example of saving
some 32-bit and some 64-bit values.
Figure 2-32 shows the standard stack frame.
Address
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r11 points here for 32-bit call -->
SP --->
r11 points to 0x388 for 64-bit call
-->
0x400
0x3FC
0x3F8
...
0x3E8
0x3E4
0x3E0
0x3DC
0x3D8
...
0x3AC
0x3A8
0x3A4
Description
Back chain
r31 word
r30 word
...
r26 word
r25 double word
r24 double word
...
r18 double word
Local variable space
Parameter save area,if any
LR save area, if any
Back chain
... ...
0x388
Figure 2-32. Standard Stack Frame
To save r26–r31 as 32-bit values, as shown in the stack frame in Figure 2-32, and r18–r25
as 64-bit values, r11 should contain 0x400 (address of the back chain word) on entry to
the appropriate 32-bit save routine (_save32gpr_26), and then be adjusted to contain 1000
- 8*25 + 104 = 904 (0x3e8 - 8 * 25 + 0x68 = 0x388) in preparation for the call to the 64-bit
save routine (_save64gpr_ctr_18). r25 will be stored at 0x388+0x58 = byte 0x3e0, which
is the double word directly preceding the 32-bit save area. Note that the r11 pointer for the
64-bit call does not necessarily point to any particularly meaningful location directly; it
points to an address such that the fixed offsets make the register saves and restores line up
appropriately. (In fact, r11 points to where r14 would be stored if it needed saving.)
2.7.3.3
Details about the Functions
Each function described in this section is a family of 18 functions with identical behavior
except for the number and kind of register affected. The function names below use the
notation [32/64] to designate the use of a 32 for the 32-bit general register functions and a
64 for the 64-bit general register functions. The suffix ‘_m’ designates the portion of the
name that would be replaced by the first register to be saved. That is, to save registers 18
through 31, one should call _save32gpr_18.
Figure 2-33 shows an example implementation.
Chapter 2. Low-Level System Information
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