NetWinder Floating Point Emulator
(c) Rebel.COM, 1998
(c) Philip Blundell 1998-1999
Direct questions, comments to Scott Bambrough <email@example.com>
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
/* This is the kernel's entry point into the floating point emulator.
It is called from the kernel with code similar to this:
mov fp, #0
teqp pc, #PSR_I_BIT | MODE_SVC
ldr r4, .LC2
ldr pc, [r4] @ Call FP module USR entry point
The kernel expects the emulator to return via one of two possible
points of return it passes to the emulator. The emulator, if
successful in its emulation, jumps to ret_from_exception and the
kernel takes care of returning control from the trap to the user code.
If the emulator is unable to emulate the instruction, it returns to
fpundefinstr and the kernel halts the user program with a core dump.
This routine does four things:
1) It saves SP into a variable called userRegisters. The kernel has
created a struct pt_regs on the stack and saved the user registers
into it. See /usr/include/asm/proc/ptrace.h for details. The
emulator code uses userRegisters as the base of an array of words from
which the contents of the registers can be extracted.
2) It locates the FP emulator work area within the TSS structure and
points `fpa11' to it.
3) It calls EmulateAll to emulate a floating point instruction.
EmulateAll returns 1 if the emulation was successful, or 0 if not.
4) If an instruction has been emulated successfully, it looks ahead at
the next instruction. If it is a floating point instruction, it
executes the instruction, without returning to user space. In this
way it repeatedly looks ahead and executes floating point instructions
until it encounters a non floating point instruction, at which time it
returns via _fpreturn.
This is done to reduce the effect of the trap overhead on each
floating point instructions. GCC attempts to group floating point
instructions to allow the emulator to spread the cost of the trap over
several floating point instructions. */
mov sl, sp
ldr r5, [sp, #60] @ get contents of PC
bic r5, r5, #0xfc000003
ldr r0, [r5, #-4] @ get actual instruction into r0
bl EmulateAll @ emulate the instruction
1: cmp r0, #0 @ was emulation successful
beq fpundefinstr @ no, return failure
.Lx1: ldrt r6, [r5], #4 @ get the next instruction and
@ increment PC
and r2, r6, #0x0F000000 @ test for FP insns
teq r2, #0x0C000000
teqne r2, #0x0D000000
teqne r2, #0x0E000000
bne ret_from_exception @ return ok if not a fp insn
ldr r9, [sp, #60] @ get new condition codes
and r9, r9, #0xfc000003
orr r7, r5, r9
str r7, [sp, #60] @ update PC copy in regs
mov r0, r6 @ save a copy
mov r1, r9 @ fetch the condition codes
bl checkCondition @ check the condition
cmp r0, #0 @ r0 = 0 ==> condition failed
@ if condition code failed to match, next insn
beq next @ get the next instruction;
mov r0, r6 @ prepare for EmulateAll()
adr lr, 1b
orr lr, lr, #3
b EmulateAll @ if r0 != 0, goto EmulateAll
.Lret: b ret_from_exception @ let the user eat segfaults
@ We need to be prepared for the instruction at .Lx1 to fault.
@ Emit the appropriate exception gunk to fix things up.
ldr lr, [lr, $(.Lret - .Lx1)/4]