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 | FCTX|  *
 *  -------------------------------------------------  *
 *  -------------------------------------------------  *
 *  |  24 |  25 |  26 |  27 |  28 |  29 |  30 |  31 |  *
 *  -------------------------------------------------  *
 *  |  96 | 100 | 104 | 108 | 112 | 116 | 120 | 124 |  *
 *  -------------------------------------------------  *
 *  | DATA|     |     |     |                       |  * 
 *  -------------------------------------------------  *
 *                                                     *
 *******************************************************/

.text
.globl _ontop_fcontext
.align 2
_ontop_fcontext:
    # reserve space on stack
    subi  r1, r1, 92

    stw  r13, 0(r1)  # save R13
    stw  r14, 4(r1)  # save R14
    stw  r15, 8(r1)  # save R15
    stw  r16, 12(r1)  # save R16
    stw  r17, 16(r1)  # save R17
    stw  r18, 20(r1)  # save R18
    stw  r19, 24(r1)  # save R19
    stw  r20, 28(r1)  # save R20
    stw  r21, 32(r1)  # save R21
    stw  r22, 36(r1)  # save R22
    stw  r23, 40(r1)  # save R23
    stw  r24, 44(r1)  # save R24
    stw  r25, 48(r1)  # save R25
    stw  r26, 52(r1)  # save R26
    stw  r27, 56(r1)  # save R27
    stw  r28, 60(r1)  # save R28
    stw  r29, 64(r1)  # save R29
    stw  r30, 68(r1)  # save R30
    stw  r31, 72(r1)  # save R31
    stw  r3,  76(r1)  # save hidden

    # save CR
    mfcr  r0
    stw   r0, 80(r1)
    # save LR
    mflr  r0
    stw   r0, 84(r1)
    # save LR as PC
    stw   r0, 88(r1)

    # store RSP (pointing to context-data) in R7
    mr  r7, r1

    # restore RSP (pointing to context-data) from R4
    mr  r1, r4

    lwz  r13, 0(r1)  # restore R13
    lwz  r14, 4(r1)  # restore R14
    lwz  r15, 8(r1)  # restore R15
    lwz  r16, 12(r1)  # restore R16
    lwz  r17, 16(r1)  # restore R17
    lwz  r18, 20(r1)  # restore R18
    lwz  r19, 24(r1)  # restore R19
    lwz  r20, 28(r1)  # restore R20
    lwz  r21, 32(r1)  # restore R21
    lwz  r22, 36(r1)  # restore R22
    lwz  r23, 40(r1)  # restore R23
    lwz  r24, 44(r1)  # restore R24
    lwz  r25, 48(r1)  # restore R25
    lwz  r26, 52(r1)  # restore R26
    lwz  r27, 56(r1)  # restore R27
    lwz  r28, 60(r1)  # restore R28
    lwz  r29, 64(r1)  # restore R29
    lwz  r30, 68(r1)  # restore R30
    lwz  r31, 72(r1)  # restore R31
    lwz  r4,  76(r1)  # restore hidden

    # restore CR
    lwz   r0, 80(r1)
    mtcr  r0
    # restore LR
    lwz   r0, 84(r1)
    mtlr  r0
    # ignore PC

    # adjust stack
    addi  r1, r1, 92

    # return transfer_t 
    stw  r7, 0(r4)
    stw  r5, 4(r4)

    # restore CTR
    mtctr r6

    # jump to ontop-function
    bctr