1. List Allocation and Garbage Collection
Example

2. Memory Allocation AVAIL = 0  (0 0 0 0 0 0 0 0) Initialize Heap1
AVAIL = 0  ( )
Initialize Heap
denotes list 2 1 3 2 4 3 5 4 6 5 7 6 -1 7

3. Memory Allocation AVAIL = 0 (define L (list 1 2 3)) 1
AVAIL = 0
(define L (list 1 2 3)) 
(cons 1 (cons 2 (cons 3 ‘()))) 2 1 3 2 4 3 5 4 6 5 7 6 -1 7

4. Memory Allocation AVAIL = 1 (define L (list 1 2 3)) -1
AVAIL = 1
(define L (list 1 2 3)) 
(cons 1 (cons 2 (cons 3 ‘()))) 2 1 3 2 4 3 5 4 6 5 7 6 -1 7

5. Memory Allocation AVAIL = 2 (define L (list 1 2 3)) -1
AVAIL = 2
(define L (list 1 2 3)) 
(cons 1 (cons 2 (cons 3 ‘()))) 2 1 3 2 4 3 5 4 6 5 7 6 -1 7

6. Memory Allocation AVAIL = 3  (0 0 0 0 0) L = 2  (1 2 3)-1
AVAIL = 3  ( )
L = 2  (1 2 3)
(define L (list 1 2 3)) 
(cons 1 (cons 2 (cons 3 ‘()))) 2 1 1 1 2 4 3 5 4 6 5 7 6 -1 7

7. Memory Allocation AVAIL = 5  (0 0 0), L = 2  (1 2 3), M = 4  (4 5)-1
AVAIL = 5  (0 0 0),
L = 2  (1 2 3),
M = 4  (4 5)
(define L (list 1 2 3))
(define M (list 4 5)) 2 1 1 1 2 5 -1 3 4 3 4 6 5 7 6 -1 7

8. Memory Allocation AVAIL = -1  NULL, L = 2  (1 2 3), M = 4  (4 5)
AVAIL = -1  NULL,
L = 2  (1 2 3),
M = 4  (4 5)
N = 7  (6 7 8)
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8)) 2 1 1 1 2 5 -1 3 4 3 4 8 -1 5 7 5 6 6 6 7

9. Memory Allocation AVAIL = -1  NULL, L = 2  ((4 5) 2 3),
AVAIL = -1  NULL,
L = 2  ((4 5) 2 3),
M = 4  (4 5)
N = 7  (6 7 8)
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M); overlap 2 1 4 1 2 5 -1 3 4 3 4 8 -1 5 7 5 6 6 6 7

10. Memory Allocation AVAIL = -1  NULL, L = 2  ((4 5) 2 3), M = 0 [int]
AVAIL = -1  NULL,
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 7  (6 7 8)
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0) ; cells still accessible 2 1 4 1 2 5 -1 3 4 3 4 8 -1 5 7 5 6 6 6 7

11. Memory Allocation AVAIL = -1  NULL, L = 2  ((4 5) 2 3), M = 0 [int]
AVAIL = -1  NULL,
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 0 [int]
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0)
(define N 0) ; cells garbage 2 1 4 1 2 5 -1 3 4 3 4 8 -1 5 7 5 6 6 6 7

12. Garbage Collection AVAIL = -1  NULL, L = 2  ((4 5) 2 3), M = 0 [int]
AVAIL = -1  NULL,
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 0 [int]
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0)
(define N 0)
(define N (list 1)); AVAIL = NULL
 garbage collection 2 1 4 1 2 5 -1 3 4 3 4 8 -1 5 7 5 6 6 6 7

13. Mark AVAIL = -1  NULL, L = 2  ((4 5) 2 3), M = 0 [int] N = 0 [int]
AVAIL = -1  NULL,
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 0 [int]
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0)
(define N 0)
(define N (list 1))
Mark L 2 1 4 1 2 5 -1 3 4 3 4 8 -1 5 7 5 6 6 6 7

14. Mark AVAIL = -1  NULL, L = 2  ((4 5) 2 3), M = 0 [int] N = 0 [int]
AVAIL = -1  NULL,
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 0 [int]
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0)
(define N 0)
(define N (list 1))
Mark L 2 1 4 1 2 5 -1 3 4 3 4 8 -1 5 7 5 6 6 6 7

15. Mark AVAIL = -1  NULL, L = 2  ((4 5) 2 3), M = 0 [int] N = 0 [int]
AVAIL = -1  NULL,
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 0 [int]
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0)
(define N 0)
(define N (list 1))
Mark L 2 1 4 1 2 5 -1 3 4 3 4 8 -1 5 7 5 6 6 6 7

16. Mark AVAIL = -1  NULL, L = 2  ((4 5) 2 3), M = 0 [int] N = 0 [int]
AVAIL = -1  NULL,
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 0 [int]
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0)
(define N 0)
(define N (list 1))
Mark L 2 1 4 1 2 5 -1 3 4 3 4 8 -1 5 7 5 6 6 6 7

17. Mark AVAIL = -1  NULL, L = 2  ((4 5) 2 3), M = 0 [int] N = 0 [int]
AVAIL = -1  NULL,
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 0 [int]
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0)
(define N 0)
(define N (list 1))
Mark L 2 1 4 1 2 5 -1 3 4 3 4 8 -1 5 7 5 6 6 6 7

18. Sweep AVAIL = 5  (0) L = 2  ((4 5) 2 3), M = 0 [int] N = 0 [int]-1
AVAIL = 5  (0)
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 0 [int]
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0)
(define N 0)
(define N (list 1))
Reclaim unmarked cells 2 1 4 1 2 5 -1 3 4 3 4 -1 5 7 5 6 6 6 7

19. Sweep AVAIL = 6  (0 0) L = 2  ((4 5) 2 3), M = 0 [int] N = 0 [int]-1
AVAIL = 6  (0 0)
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 0 [int]
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0)
(define N 0)
(define N (list 1))
Reclaim unmarked cells 2 1 4 1 2 5 -1 3 4 3 4 -1 5 5 6 6 6 7

20. Sweep AVAIL = 7  (0 0 0) L = 2  ((4 5) 2 3), M = 0 [int] N = 0 [int]-1
AVAIL = 7  (0 0 0)
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 0 [int]
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0)
(define N 0)
(define N (list 1))
Reclaim unmarked cells 2 1 4 1 2 5 -1 3 4 3 4 -1 5 5 6 6 7

21. Sweep AVAIL = 7  (0 0 0) L = 2  ((4 5) 2 3), M = 0 [int] N = 0 [int]-1
AVAIL = 7  (0 0 0)
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 0 [int]
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0)
(define N 0)
(define N (list 1))
Unmark cells 2 1 4 1 2 5 -1 3 4 3 4 -1 5 5 6 6 7

22. Memory Allocation AVAIL = 6  (0 0) L = 2  ((4 5) 2 3), M = 0 [int]-1
AVAIL = 6  (0 0)
L = 2  ((4 5) 2 3),
M = 0 [int]
N = 7  (1)
(define L (list 1 2 3))
(define M (list 4 5))
(define N (list 6 7 8))
(set-car! L M)
(define M 0)
(define N 0)
(define N (list 1)) 2 1 4 1 2 5 -1 3 4 3 4 -1 5 5 6 1 -1 7