Programming task “PseudoVM”

NOTE: Need to cover exceptions first to be able to solve this task.

See program PseudoVM.s.

Write an exception handler that imitates “virtual memory” for “forbidden” addresses. A “forbidden” address is any address that causes exceptions LOAD_ACCESS_FAULT and STORE_ACCESS_FAULT when we try to access it (read or write). This is not supported for address 0x0 (it is reserved).

It is suggested to create a table (array) that will store records "virtual address":value (pairs of 4-byte values). The capacity of the table is 16 records (i.e. 2*4*16=128 bytes). Address 0x0 can be used to specify an empty record.

“Virtual memory” works only with instructions lw and sw that use register t0 as a source/destination for values (other registers are not checked).

Reading from an address works in the following way:

• If the address is present in the table, the value stored in the table is returned.
• If the address is missing from the table, 0 is returned.

Writing to an address works in the following way:

• If the address is present in the table, the value stored in the record is updated.
• If the address is missing from the table, but the table has free records, a new record "virtual address":value is placed into the table.
• If the address is missing from the table and its full (no free records), nothing happens.

Notes:

• Everything is done in the handler (starts with the handler label) that handles the two exceptions.
• The handler must save and restore all registers it uses (some area in the .data section).
• This main program will be merged with the handler (you must submit only the handler).
• The main program reads addresses from user input: addresses multiple of 4 are used for reading, others - for writing.
• Examples of an input and output for the program are below.

Input:

21
123
22
1234
20
1001
100500
1000
100
-70001
-70001
-70000
-70004
0

Output:

1234
100500
0
0
-70001