Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS VOLUME 3 REV 2.3, Manual

Page: 804 / 1898

2:556 Volume 2, Part 2: Context Management
the
epc
until the switch to the kernel backing store has been completed. Additionally,
low-level operating system handlers should not only use IPSR.cpl, but should also
check BSPSTORE, to determine whether they are running on the kernel backing store
(imagine an external interrupt being delivered on the first instruction after the
epc
).
4.4.2 break/rfi
The
break
instruction, when issued in the i , f , and m syllables, specifies an arbitrary
21-bit immediate value. The kernel can choose a specific
break
immediate value to
differentiate system calls from other usage of the
break
instruction (such as debug).
The
break
instruction jumps to the
break
fault handler, which should be a valid address
mapping for each user application, and raises the privilege mode to the most privileged
level.
The system call number is an additional parameter passed to the kernel when invoking
a system call via the
break
instruction. The system call number must reside in a fixed
location. If stored within GR32, then the system call stub must rearrange its input
parameters to map to the register stack starting at GR33. This register jostling can be
avoided by passing the system call number through a scratch static general register or
by using the
break
immediate itself. Additionally, the system call can utilize all eight
input registers of the register stack for system call parameters.
4.4.3 NaT Checking for NaTs in System Calls
In addition to regular range/value checking on system call arguments, Itanium
architecture-based operating systems need to additionally ensure that system call
arguments passed in by a user application do not have any NaT bits set. The following
code fragment can be used:
mov mask = 0xff
clrrrb
;;
// create register stack frame with only output registers for system call args
alloc tmp = ar.pfs, 0, 0, 8, 0
shl mask = mask, syscall_arg_count
;;
mov pr = mask, 0xff00 // define p8 .. p15
;;
cmp.eq p7 = r0, r0 // set p7 to true
;;
// test for NaT bits in the input arguments
(p8) cmp.eq.and p7 = r32, r32 // and type compare clears p7 if r32 is NaT
(p9) cmp.eq.and p7 = r33, r33
(p10) cmp.eq.and p7 = r34, r34
(p11) cmp.eq.and p7 = r35, r35
(p12) cmp.eq.and p7 = r36, r36
(p13) cmp.eq.and p7 = r37, r37
(p14) cmp.eq.and p7 = r38, r38
(p15) cmp.eq.and p7 = r39, r39
(p7) br.cond.sptk ok_arguments // No NaTs found
;;
// p7 was cleared by at least one NaT argument