add L6.pdf and complement in 10.30

notes/6.md

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+# Code Generation 1
+
+## Organization of Storage
+
+* Code Area
+* Static Area
+* Stack Area
+* Heap Area
+
+### Stack Management using Register File
+
+* Stack pointer (SP): points to the top of the frame
+* Frame Pointer (FP, BP): points to the frame base
+
+because stack frame size is not always known at compile time.
+
+
+### Saving Registers
+
+The callee may overwrite useful values in the registers.
+
+So caller's useful values should be stored in stack when function-call (`push`), and restored when callee return (`pop`). This role is done by both callee and caller (`caller-saved`, `callee-saved`).
+
+* Caller-saved registers
+used to hold **temporary quantities** that need not be preserved across calls.
+* Callee-saved registers
+used to hold **long-lived values** that should be preserved across calls.
+
+
+So Code before call:
+* push the params.
+* push caller-saved registers
+* push return address (curr `PC`) and jump to callee code
+
+Prologue of callee
+* push dynamic link (`FP`)
+* old stack pointer becomes new frame pointer
+* push callee-saved registers
+* push local var
+
+Epilogue
+* pop callee-saved registers
+* store return value at appropriate place
+* restore old stack pointer
+* pop old frame pointer
+* pop return addr and jump to that
+
+Code after call
+* pop caller-saved registers
+* use return value from appropriate place
+
+When accessing stack variable, use offsets from `FP`.
+
+## Global Variables or Static Variables
+
+Global variables have same references for all scope. They are assigned a **fixed address once** (**statically allocated**).
+
+`Code > Static Data > Stack`
+
+
+## Dynamic Allocation
+
+The dynamically allocated value outlives the procedure creating it (unless deleted).
+
+`Code > Static Data > Stack -> <- Heap`
+
+
+