context 2.0.1

Cooperative multitasking for Rust using Boost.Context
Documentation 
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/*
            Copyright Oliver Kowalke 2009.
   Distributed under the Boost Software License, Version 1.0.
      (See accompanying file LICENSE_1_0.txt or copy at
          http://www.boost.org/LICENSE_1_0.txt)
*/

/******************************************************
 *                                                     *
 *  -------------------------------------------------  *
 *  |  0  |  1  |  2  |  3  |  4  |  5  |  6  |  7  |  *
 *  -------------------------------------------------  *
 *  |  0  |  4  |  8  |  12 |  16 |  20 |  24 |  28 |  *
 *  -------------------------------------------------  *
 *  | R13 | R14 | R15 | R16 | R17 | R18 | R19 | R20 |  *
 *  -------------------------------------------------  *
 *  -------------------------------------------------  *
 *  |  8  |  9  |  10 |  11 |  12 |  13 |  14 |  15 |  *
 *  -------------------------------------------------  *
 *  |  32 |  36 |  40 |  44 |  48 |  52 |  56 |  60 |  *
 *  -------------------------------------------------  *
 *  | R21 | R22 | R23 | R24 | R25 | R26 | R27 | R28 |  *
 *  -------------------------------------------------  *
 *  -------------------------------------------------  *
 *  |  16 |  17 |  18 |  19 |  20 |  21 |  22 |  23 |  *
 *  -------------------------------------------------  *
 *  |  64 |  68 |  72 |  76 |  80 |  84 |  88 |  92 |  *
 *  -------------------------------------------------  *
 *  | R29 | R30 | R31 |hiddn|  CR |  LR |  PC |bchai|  *
 *  -------------------------------------------------  *
 *  -------------------------------------------------  *
 *  |  24 |  25 |  26 |  27 |  28 |  29 |  30 |  31 |  *
 *  -------------------------------------------------  *
 *  |  96 | 100 | 104 | 108 | 112 | 116 | 120 | 124 |  *
 *  -------------------------------------------------  *
 *  |linkr| FCTX| DATA|                             |  * 
 *  -------------------------------------------------  *
 *                                                     *
 *******************************************************/

.text
.globl make_fcontext
.align 2
.type make_fcontext,@function
make_fcontext:
    # save return address into R6
    mflr  %r6

    # first arg of make_fcontext() == top address of context-function
    # shift address in R3 to lower 16 byte boundary
    clrrwi  %r3, %r3, 4

    # reserve space for context-data on context-stack
    # including 64 byte of linkage + parameter area (R1 % 16 == 0)
    subi  %r3, %r3, 172

    # third arg of make_fcontext() == address of context-function
    stw  %r5, 88(%r3)

    # set back-chain to zero
    li   %r0, 0
    stw  %r0, 92(%r3)

    # compute address of returned transfer_t
    addi  %r0, %r3, 100
    mr    %r4, %r0 
    stw   %r4, 76(%r3) 

    # load LR
    mflr  %r0
    # jump to label 1
    bl  1f
1:
    # load LR into R4
    mflr  %r4
    # compute abs address of label finish
    addi  %r4, %r4, finish - 1b
    # restore LR
    mtlr  %r0
    # save address of finish as return-address for context-function
    # will be entered after context-function returns
    stw  %r4, 84(%r3)

    # restore return address from R6
    mtlr  %r6

    blr  # return pointer to context-data

finish:
    # save return address into R0
    mflr  %r0
    # save return address on stack, set up stack frame
    stw  %r0, 4(%r1)
    # allocate stack space, R1 % 16 == 0
    stwu  %r1, -16(%r1)

    # exit code is zero
    li  %r3, 0
    # exit application
    bl  _exit@plt
.size make_fcontext, .-make_fcontext

/* Mark that we don't need executable stack.  */
.section .note.GNU-stack,"",%progbits