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improved code comments about restore/save context of task

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Signed-off-by: Wang Chen <wangchen20@iscas.ac.cn>
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Wang Chen
2023-02-12 13:23:20 +08:00
parent c674ffc9bc
commit f77939f1e8
9 changed files with 37 additions and 19 deletions
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8
code/os/04-multitask/entry.S
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@@ -77,9 +77,11 @@
.endm
# Something to note about save/restore:
# - We use mscratch to hold a pointer to context of previous task
# - We use mscratch to hold a pointer to context of current task
# - We use t6 as the 'base' for reg_save/reg_restore, because it is the
# very bottom register (x31) and would not be overwritten during loading.
# Note: CSRs(mscratch) can not be used as 'base' due to load/restore
# instruction only accept general purpose registers.
.text
@@ -89,7 +91,7 @@
.align 4
switch_to:
csrrw t6, mscratch, t6 # swap t6 and mscratch
beqz t6, 1f # Notice: the first time switch_to() is
beqz t6, 1f # Note: the first time switch_to() is
# called, mscratch is initialized as zero
# (in sched_init()), which makes t6 zero,
# and that's the special case we have to
@@ -98,7 +100,7 @@ switch_to:
# Save the actual t6 register, which we swapped into
# mscratch
mv t5, t6 # t5 points to the context of next task
mv t5, t6 # t5 points to the context of current task
csrr t6, mscratch # read t6 back from mscratch
sw t6, 120(t5) # save t6 with t5 as base