Write an assembler in Jsva (Comment Heavily) that reads the source code of an assembly program (for an imaginary machine) and displays the machine language for that program.

The following assembler instructions must be supported.

Instruction	opcode	Length	Format	Example
add	0	2	0	add R1, R2, R3	R3 = R1 + R2
sub	1	2	0	sub R1, R2, R3	R3 = R1 - R2
mult	2	2	0	mult R1, R2, R3	R3 = R1 * R2
div	3	2	0	div R1, R2, R3	R3 = R1 / R2
load	4	4	1	load R1,addr[R7]	R1 memory value
laddr	5	4	1	laddr R1,addr[R7]	R1 address
store	6	4	1	store R1,addr[R7]	R1 memory
call	7	4	2	call addr	push return addr on stack and jump to addr
rtn	8	1	4	rtn	pop address from stack and jump to there
jump	9	4	2	jump addr[R7]	jump to address
jz	A	4	1	jz R1,addr[R7]	jump to address only if R1 is zero
jneg	B	4	1	jn R1,addr[R7]	jump to address only if R1 is negative
push	C	1	3	push R1	push R1 on stack
pop	D	1	3	pop R1	pop stack into R1
lpsw	E	1	3	lpsw R1	load PSW into R1
spsw	F	1	3	spsw R1	store R1 into PSW
data		4	5	data 123	created a data word

Machine language instructions have the following formats

: 0: arithmetic

opcode	Reg 1	Reg 2	Reg 3
4 bits	4 bits	4 bits	4 bits

1: Load, store, conditional jump

opcode	Reg	index	addr
4 bits	4 bits	4 bits	20 bits

2: call, jump

opcode	unused	index	addr
4 bits	4 bits	4 bits	20 bits

3: push, pop, lpsw, spsw

opcode	Reg
4 bits	4 bits

4: rtn

opcode	unused
4 bits	4 bits

5: data

integer data value

32 bits

A comment in the assembly program is indicated by an asterisk, "*", in the first column of the source code. Labels must start in the first column. Your assembler does not have to detect every possible error in the assembler program, but it should produce a reasonable error message if an undefined label is used.

For a maximum score of 95 points, your program can ignore all index registers. The presence of an [index register] may or may not be considered an error. The index field of all instructions must be set to zero if index registers are not implemented. For a maximum score of 100, all memory addressing instructions must support an optional index register.

Hints:

 e 2324 10 b400000c 14 1415 16 a300001c 1a 3536 1c 66000025 nodiv store R6, addr 20 c6 push R6 

21 7000002d call mthd 

25 26 28 29 9000000c 

2d 2f

30 34 38

 * example method 128 mthd add R1, R2, R8 

80 rtn * data

1 dog data 1

0cat data0 801 goat data 2049

Example output:

 address machine source 

 * Example assembler program 

0 58000034 4 41800030 8 42800038 c 123 more add R1, R2, R3 

laddr R8,cat load R1, dog[R8] load R2, goat[R8] 

mult R3, R2, R4 jn R4, more sub R4, R1, R5 jz R3, nodiv div R5, R3, R6 

fa addr spsw R10 

a27 e7

add R10, R2, R7 lpsw R7 jump more 

You may wish to create an object to hold the symbol table entries. This would contain:

• opcode, address or other value of the symbol

• length of instruction in bytes

• format of the machine instruction

  A hash table works well to hold the symbol table. Symbol table entries can be keyed by the mnemonic or address label.

  The assembler can be written to use one or two passes. After the first pass, a two pass assembler closes the input file and then reads the input again. The goal of the first pass is to create a symbol table containing the address of all symbols. The second pass creates the output. It uses the symbol table to get the value of all names used in the program.

  General outline for pass 1.

• Initialize the current address to zero

• Read a line of assembler

• Split the line into tokens

• If label, put label and current address in symbol table

• Get mnemonic and find in symbol table

• Add instruction length to current address

• repeat until end of file

  General outline for pass 2

• Initialize the current address to zero

• Read a line of assembler

• Split the line into tokens

• Get mnemonic information from symbol table

• Get additional fields and create the machine language

• Display the results

  The following Java classes and methods may be useful in writing your assembler. .

  java.util.HashMap java.util.StringTokenizer Integer.toUnsignedString( intNum, 16)