C-Course-Assembler-Project

The final assignment (#14) of the C course (20465) at The Open University.
It received a grade of 100.

This project is based on the two-pass assembler model.
Note: the computer model for this project and the given assembly language are imaginary.

Getting Started

The project was coded and compiled using Ubuntu, but it may run on all Linux versions.

Usage

Use makefile to compile the project like this:

>   make

After preparing assembly files with an .as extension, open terminal and pass file names as arguments (without the file extensions) as following:

As for the files x.as, y.as, hello.as we will run:

>   assembler x y hello

The assembler will generate output files with the same filenames and the following extensions:

• .ob - Object file
• .ent - Entries file
• .ext - Externals file

An example of input and output files can be found under the 'tests' folder.

Hardware

• CPU

• RAM (including a stack), with the size of 256 words.

• A word's size in memory is 10 bits.

• Uses signed 2's complement arithmetic for integers (with no support for real numbers).

  Registers

  The CPU has 9 registers: 8 general registers (named r1,r2,...,r8) and a register called PSW (program status word).
  The size of each register is a word (10 bits).

Instruction Structure

Every software instruction is encoded into a few words in memory (max is 5 words). The first word is of the following structure:

9 8 7 6	5 4	3 2	1 0
Opcode	Source operand addressing method	Destination operand addressing method	E, R, A

Encoding of each instruction word is done using an imaginary base32 as defined here:

0: !   
1: @
2: #
3: $
4: %
5: ^
6: &
7: *
8: <
9: >
10: a
11: b
12: c
13: d
14: e
15: f
16: g
17: h
18: i
19: j
20: k
21: l
22: m
23: n
24: o
25: p
26: q
27: r
28: s
29: t
30: u
31: v

so that each word can be encoded as 2 digits in this base (with leading ! as needed).

Commands

The commands allowed in bits 6-9 are:

Opcode (decimal)	Command Name
0	mov
1	cmp
2	add
3	sub
4	not
5	clr
6	lea
7	inc
8	dec
9	jmp
10	bne
11	red
12	prn
13	jsr
14	rts
15	stop

Directives

A directive line of the following structure:

1. An optional preceding label. e.g. PLACE1: .

2. A directive: .data, .string, .struct, .entry or .extern.

3. Operands according to the type of the directive.

   .data

   This directive allocates memory in the data image for storing received integers later in memory (also increases the data counter and updates the symbol table). The parameters of .data are valid integers (separated by a comma).
   e.g. LABEL1: .data +7, -56, 4, 9.

   .string

   This direcive receives a string as an operand and stores it in the data image. It stores all characters by their order in the string, encoded ny their ASCII values. e.g. STRING1: .string "abcdef" is a valid directive.

   .struct

   This directive allocates memory for a structure consists of 2 fields: the first is an integer and the second is a string. e.g. STRUCT4: .struct 4, "abc" is a vaild directive.

   .entry

   This directive outputs a received name of a label to the symbol table, so that later it will be recognized by other assembly files (and they would be able to use it). e.g.

   ; file1.as
   .entry HELLO
   HELLO: add #1, r1 
   

   .extern

   This directive receives a name of a label as a parameter and declares the label as being external (defined in another file) and that the current file shall use it.
   This way, the directive .extern HELLO in file2.as will match the .entry directive in the previous example.