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Assume we have a free space management system as described in the book, for example one used to provide us with virtual addresses on the heap (with physical addresses handled by a much simpler system since we assume paging). Assume the system uses the following very simple rules;
- A memory request traverses the free list, and finds the first block large enough to handle the request (i.e., the first fit
algorithm from the book). It then:- Splits the block into 2 pieces: The first piece will be the requested size + 1k (for a header). The second will be the remaining
- The first piece will be returned to the caller, but the address returned will be the address of the memory for the user, that is the header + 1k
- The second will be put back into the linked list
- If there's insufficient contiguous memory, the allocation will fail (return 0)
- Free memory is stored as a linked list in address order
Assume we start with a single contiguous block of FREE memory of size 64k starting at location 32k in memory. Consider the following sequence of mallocs and frees:
| 1 | x = malloc(7168); /* 7k */ |
| 2 | y = malloc(5120); /* 5k */ |
| 3 | z = malloc(11264); /*11k */ |
| 4 | free(x); |
| 5 | x = malloc(8192); /* 8k */ |
| 6 | free(y); |
| 7 |
y = malloc(12288); /* 12k */ |
| 8 | free(z); |
| 9 | z = malloc(38912); /* 38k */ |
Please fill in the requested addresses. Note that since everything is in k, you may use raw numbers (e.g., 1024) or the equivalent k (e.g., 1k).
The address returned to the caller for x in line 1?
The address returned to the
The address returned to the program for z in line 3?
The address reclaimed by the OS in line 4 (hint: This may not be the same as the address of x)?
The address returned to the program for x in line 5?
The address reclaimed by the OS in line 6 (hint: This may not be the same as the address of y)?
The address returned to the program for y in line 7?
The address reclaimed by the OS in line 8 (hint: This may not be the same as the address of z)?
The address returned to the caller for z in line 9?
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this is what my professor said:
1. Memory starts at 32k, but we always have a leading 1k header, so 33k will be returned. Your reclaim (in 4) however, will reclaim the header too, so that will be 32k. You missed this. In step ~5 however, you state that there's room for 8k at 32, but you actually need 9k (8k+1) and don't have it.
Can someone please fix this and give me all the answers again?
In the previous answers:
Isn't 1 is supposed to be subtracted from the 64 as well?
-
Malloc for x:
- Requested size: 10k
- Address returned to the caller: 32k + 1k = 33k (since x will be placed after the header of 1k)
- Address returned to the program: 33k
- Remaining free memory: 54k (64k - 10k) (here)
- Address of the remaining free memory: 43k
and
Here, the requested size is 20, why is 10 subtracted?
-
Malloc for y (again):
- Requested size: 20k
- Address returned to the caller: 44k (previous address)
- Address returned to the program: 44k (previous address)
- Remaining free memory: 54k (64k - 10k) (here)
- Address of the remaining free memory: 64k
The address returned to the caller of z on line 9 is 33792 (beginning of free memory block) + 1k (header) = 34816.
this doesn't follow the line 5 and line 7.
Could you please explain why 33792 returned to in line 9 and 32k in line 5 and 7?
thank you
this is what my professor said:
1. Memory starts at 32k, but we always have a leading 1k header, so 33k will be returned. Your reclaim (in 4) however, will reclaim the header too, so that will be 32k. You missed this. In step ~5 however, you state that there's room for 8k at 32, but you actually need 9k (8k+1) and don't have it.
Can someone please fix this and give me all the answers again?
In the previous answers:
Isn't 1 is supposed to be subtracted from the 64 as well?
-
Malloc for x:
- Requested size: 10k
- Address returned to the caller: 32k + 1k = 33k (since x will be placed after the header of 1k)
- Address returned to the program: 33k
- Remaining free memory: 54k (64k - 10k) (here)
- Address of the remaining free memory: 43k
and
Here, the requested size is 20, why is 10 subtracted?
-
Malloc for y (again):
- Requested size: 20k
- Address returned to the caller: 44k (previous address)
- Address returned to the program: 44k (previous address)
- Remaining free memory: 54k (64k - 10k) (here)
- Address of the remaining free memory: 64k
The address returned to the caller of z on line 9 is 33792 (beginning of free memory block) + 1k (header) = 34816.
this doesn't follow the line 5 and line 7.
Could you please explain why 33792 returned to in line 9 and 32k in line 5 and 7?
thank you
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