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Instruction Cycle
- An instruction is fetched from the memory address
stored in the program counter (PC) and is stored in theinstruction register (IR). The PC is incremented.
- The decoder interprets the instruction. If it is a jump,
the PC is reset and the cycle ends. Otherwise, anyrequired data is fetched from main memory and isplaced in data registers.
- The CPU executes the instruction by reading values
from registers, performing arithmetic or logicalfunctions on them, and writing the result into a register.
- The result is stored in main memory or is sent to an
output device.
- Interrupts are handled.
Instruction Cycle
Call Stack
Function calls are coordinated using a
call stack
Each function call generates a
stack frame
that stores
its arguments, local variables, space for return values,and a return address. When
f
calls
g
, a
g
-frame is initialized and control is
transferred to the first instruction of
g
When
g^
returns, execution of
f
resumes at the
g
return
address. Nested function calls are handled by processing stackframes in first-in first-out order (FIFO).
Call Stack
drawline
arguments local variables^ return value return address
drawbox
arguments local variables^ return value return address
Recursive Functions
A^
recursive
function contains calls to itself in its body.
These calls are regular function calls. The programmer must ensure that the calls terminate. Example: compute
1 + 2 +
· · ·
n
def sum (n):
if
n == 1:
return
return
n +
sum(n - 1)
Base case
of
n
= 1
has no recursion.
Recursive calls terminate because
n
decreases.
Call Stack
sum(1)
local variable:
n
= 1
return value: 1
return address:
<
sum(2)
sum(2)
local variable:
n
= 2
return value:
2 + sum(1)
return address:
<
sum(3)
sum(3)
local variable:
n
= 3
return value:
3 + sum(2)
return address:
<
sum(4)
sum(4)
local variable:
n
= 4
return value:
4 + sum(3)
return address:
<
main
