add L7 and L8 pdf and complement in 1103

notes/8.md

@@ -0,0 +1,86 @@
+# ARM Subroutines
+
+Subroutine, Function, Procedure
+
+It has a single-entry and single-exit.
+
+* `bl lbl`
+  * set `lr` to the next instruction location
+  * set `pc` to the label
+* `bx lr`
+  * set `pc` to the saved `lr`
+
+ARM register calling convention:
+
+
+
+Passing Argument via registers:
+
+```armasm
+    mov r0, #3
+    mov r1, #4
+    bl ssq
+    mov r2, r0
+    b endl
+SSQ:
+    mul r0, r0, r0
+    mul r1, r1, r1
+    add r0, r0, r1
+    bx lr
+```
+
+`pc` **is always incremented by 4**. Each time, 4 bytes are fetched from the instruction memory. It is not depending on instruction size (2 or 4 bytes).
+
+
+## Using Stack
+
+Stack is a LIFO data structure.
+* `push` / `pop`
+
+There is two stack growth convention:
+* Ascending and Descending.
+* Full and Empty
+
+`sp` = `r13`
+Coretex-M uses **Full Descending stack**.
+
+commands
+* `push {register_list}`
+* `stmdb sp!, {register_list}`
+* `pop {register_list}`
+* `ldmia sp!, {register_list}`
+
+**`push` instruction**
+
+`push {Rd}`
+
+**`pop` instruction**
+
+
+### multi-level subroutines
+
+```armasm
+main:
+    mov r0, #2
+    bl quad
+quad:
+    push { lr }
+    bl sq
+    bl sq
+    pop { lr }
+    bx lr
+sq:
+    mul r0, r0
+    bx lr
+```
+
+## Initializing SP
+
+Before using the stack, software has to define stack pointer
+
+
+## Recursive Function
+
+No difference to multi-level subroutines.
+
+It has multiple stack frames.