A simple SBNZ (Subtract, Branch If Not Zero) VM Interpreter implemented in Clojure.
$ lein run program.sbnz
$ lein run program.sbnz memory.txt
Takes one or two arguments. First argument is a SBNZ source code file, the optional second argument is a filename that will contain a memory dump of the VM memory image in the form of space separated integers.
Source code stored in a file named program.sbnz
SBNZ [0] [0] a 4
SBNZ [0] [7] b 8
SBNZ [0] [-9] c 12
SBNZ c a a 16
SBNZ [1] b b 1024
SBNZ [0] [0] [0] 12
OUTPUT a
OUTPUT b
OUTPUT c
Run SBNZ program, whose purpose is to calculate 7 (stored in variable b) times 9 (stored in variable c) and store the result in variable a. The result of this computation is 63 as expected. The output shows that b's original value of 7 has been decremented to 0 in a loop that has executed 7 times, and that c is initialized to -9 so that a - c results in a - (-9), or equivalentlya + 9.
$ lein run program.sbnz
63
0
-9
The memory space for the VM is 0-1023. Using a jump address less than 0 or greater than 1023 (like 1024 in the program above) will be interpreted as end of program execution
The SBNZ instructions are composed of 4 memory references which can be of the following types
| Reference type | Description |
|---|---|
| <number> | A direct memory cell reference (0, 4, 1024, etc.) |
| <variable name> | A reference to a memory cell whose address will be computed (a, b, c, foo, bar, etc.) |
| [<number>] | An integer that will be stored in a memory cell whose address will be computed, ([0], [1], [7], etc.) |
The OUTPUT directive takes one argument that can be one of the following types
| Reference type | Description |
|---|---|
| <number> | A direct memory cell reference |
| <variable name> | A reference to a memory cell whose address will be computed |
Copyright 2020 Lars Nilsson
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