CS212 Spring 2002
3/13/2002
Lecture 8: Introduction to Part 3 (continued)
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[0] Announcements:
+ point break down so far: 5% part 1, about 15% part 2
+ Part 3 posted later tonight/thurs afternoon
  - add control structures to compiler
  - sam-code for functions (not part of compiler -- that's in Part 4)
+ late assignments dealt with case-by-case (grades might be delayed)
+ reading: see Lecture 7 (posted with answers), SaM.pdf in lecture notes
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[1] Summary:
+ function calls: frames in stack, collection of frames is activation record
+ sam-code for functions:
  need to "remember" current position and last position in stack
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[2] Overview:
+ review of FBR and frames in SaM
+ partial example of function call
+ complete sam-code for functions (basically, the "rest" of sam-code)
+ more examples
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[3] Complete sam-code for the example in [7] (Lecture 7) (w/o main)

+ sam-code reminders:
  STOREOFF x :  V[x+FBR] <- V[top]    (store variable value)
  PUSHOFF x  :  push V[x+FBR]         (retrieve variable value)
  PUSHFBR    :  push FBR onto stack   (push current FBR)
  POPFBR     :  FBR <- V[top]         (store new FBR)
  JSR t      :  push PC+1; PC <- t    (store return address; jump to function)
  JUMPIND    :  PC <- V[top]          (go back to point where function called)

+ we're writing sam-code for a call of "f(10,20)"
  where "f(int x,int y) { return x+y; }"  (I added types)

  // sam-code from above
  // now dealing with current function call
  PUSHIMM 0   // leave space for rv
  PUSHFBR     // push the current value of FBR
  PUSHIMM 10  // push param
  PUSHIMM 20  // push param
  PUSHSP   ----
  PUSHIMM 3    \_____ set up FBR: FBR now points to bottom of current frame
  SUB          /      (SP-params+1 gives new FBR)
  POPFBR   ----
  JSR f       // jump to function (jump to callee)
  ADDSP -2    // remove params
  POPFBR      // restore FBR (the FBR gets its previous value)
  f: // code for callee (the function that gets called)
  // ADDSP n not needed because no local vars besides formal params
  // you could say ADDSP 0 for completeness, which is a bit better
  PUSHOFF 1   // get x
  PUSHOFF 2   // get y
  STOREOFF -1 // store x+y in rv
  JUMPIND     // return to caller






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[4] Generic sam-code:

PROGRAM:
// loading starts at address 0
PUSHIMM 0
PUSHFBR
PUSHIMM 1
POPFBR
JSR main
POPFBR
STOP
// code for all other functions, starting with main


FUNCTION:
// label of first command is function name
label:       
// allocate local variables (params already taken care of!)
ADDSP n  
// code in body
// return code (see next segment):
STOREOFF -1
// pop off local variables:
ADDSP -n
// return to caller
JUMPIND


RETURN CODE (giving a bit more detail):
// evaluate return value
// store return value into "bottom" slot
STOREOFF -1 // takes into account that the FBR is 1 "higher" than rv
// pop off local variables:
ADDSP -n
JUMPIND


FUNCTION INVOCATION (calling function f):
// make space for rv and saved FBR
PUSHIMM 0 // or ADDSP 1
PUSHFBR
// push params
// set up new FBR for n params:
PUSHSP
PUSHIMM n+1
SUB
POPFBR
// call function f
JSR f
// pop params and restore "old" FBR (move FBR to previous saved FBR)
ADDSP -n
POPFBR










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