Doc fixes.

[bertos.git] / kern / switch_avr.S

diff --git a/kern/switch_avr.S b/kern/switch_avr.S
old mode 100755 (executable)
new mode 100644 (file)
index 08dfbe8..9389291
--- a/kern/switch_avr.S
+++ b/kern/switch_avr.S
@@ -16,12 +16,25 @@
 
 #include <avr/io.h>
 
+/*
+ * NOTE: At each change of this function affecting proc.c
+ * (i.e. arguments, data stored in the stack) bump up version
+ * number in asm_switch_version().
+ */
+
 /* void asm_switch_context(void **new_sp, void **save_sp) */
 .globl asm_switch_context
 asm_switch_context:
 
-;      push    r0      caller-save
-;      push    r1      caller-save
+;      r0 is the TEMP REG and can be used freely.
+;      r1 is the ZERO REG and must always contain 0.
+;
+;      Stack frame is 19 byte, remember to update
+;      CPU_SAVED_REGS_CNT if you change pushed regs.
+
+       in      r0,SREG-__SFR_OFFSET
+       push    r0
+;      push    r1      ;zero-reg
        push    r2
        push    r3
        push    r4
@@ -38,48 +51,58 @@ asm_switch_context:
        push    r15
        push    r16
        push    r17
-;      push    r18     caller-save
-;      push    r19     caller-save
-;      push    r20     caller-save
-;      push    r21     caller-save
-;      push    r22     caller-save
-;      push    r23     caller-save
-;      push    r24     caller-save
-;      push    r25     caller-save
-;      push    r26     caller-save
-;      push    r27     caller-save
+;      push    r18     ;caller-save
+;      push    r19     ;caller-save
+;      push    r20     ;caller-save
+;      push    r21     ;caller-save
+;      push    r22     ;caller-save
+;      push    r23     ;caller-save
+;      push    r24     ;caller-save
+;      push    r25     ;caller-save
+;      push    r26     ;caller-save
+;      push    r27     ;caller-save
        push    r28
        push    r29
-;      push    r30     caller-save
-;      push    r31     caller-save
+;      push    r30     ;caller-save
+;      push    r31     ;caller-save
 
-; First parameter (new_sp) is in r24:r25, second (save_sp) in r22:r23
+;      First parameter (new_sp) is in r24:r25, second (save_sp) in r22:r23
 
-       in      r0,SPL-__SFR_OFFSET     ; r0:r1 = SP
-       in      r1,SPH-__SFR_OFFSET
+       in      r18,SPL-__SFR_OFFSET    ; r18:r19 = SP
+       in      r19,SPH-__SFR_OFFSET
        movw    r26,r22                 ; X = save_sp
-       st      X+,r0                   ; *save_sp = SP
-       st      X,r1
+       st      X+,r18                  ; *save_sp = SP
+       st      X,r19
        movw    r26,r24                 ; X = new_sp
-       ld      r0,X+
-       ld      r1,X
-       out     SPL-__SFR_OFFSET,r0     ; SP = *new_sp
-       out     SPH-__SFR_OFFSET,r1
+       ld      r18,X+
+       ld      r19,X
+
+;      Set new stack pointer.
+;      AVR is an 8 bit processor so
+;      care must be taken when updating
+;      SP that is a 16 bit reg.
+;      Two instructions are required to update SP
+;      so an IRQ can sneak in between them.
+;      So IRQ *MUST* be disabled and then restored.
+       cli                             ; Disable interrupt
+       out     SPL-__SFR_OFFSET,r18    ; SP = *new_sp
+       out     SPH-__SFR_OFFSET,r19
+       out     SREG-__SFR_OFFSET,r0    ; Restore previous IRQ state
 
-;      pop     r31     caller-save
-;      pop     r30     caller-save
+;      pop     r31     ;caller-save
+;      pop     r30     ;caller-save
        pop     r29
        pop     r28
-;      pop     r27     caller-save
-;      pop     r26     caller-save
-;      pop     r25     caller-save
-;      pop     r24     caller-save
-;      pop     r23     caller-save
-;      pop     r22     caller-save
-;      pop     r21     caller-save
-;      pop     r20     caller-save
-;      pop     r19     caller-save
-;      pop     r18     caller-save
+;      pop     r27     ;caller-save
+;      pop     r26     ;caller-save
+;      pop     r25     ;caller-save
+;      pop     r24     ;caller-save
+;      pop     r23     ;caller-save
+;      pop     r22     ;caller-save
+;      pop     r21     ;caller-save
+;      pop     r20     ;caller-save
+;      pop     r19     ;caller-save
+;      pop     r18     ;caller-save
        pop     r17
        pop     r16
        pop     r15
@@ -96,7 +119,17 @@ asm_switch_context:
        pop     r4
        pop     r3
        pop     r2
-;      pop     r1      caller-save
-;      pop     r0      caller-save
+;      pop     r1      ;zero-reg
+       pop     r0
+       out     SREG-__SFR_OFFSET,r0
 
        ret
+
+
+/* int asm_switch_version(void) */
+.globl asm_switch_version
+asm_switch_version:
+       ldi r24,lo8(1)
+       ldi r25,hi8(1)
+       ret
+