The purpose of this lab is to reinforce the concepts of assembly language and assemblers. In this lab assignment, you will write an LC-3b Assembler, whose job will be to translate assembly language source code into the machine language (ISA) of the LC-3b. You will also write a program to solve a problem in the LC-3b Assembly Language.
In Lab Assignment 2, we will close the loop by having you complete the design of a simulator for the LC-3b, and test your assembler by having the simulator execute the program you wrote and assembled in this lab.
label opcode operands ; commentsThe leftmost field on a line will be the label field. Valid labels contain a maximum of 10 characters and do not begin with a '#' or 'x'. The label is optional, i.e., the label can be empty for some lines if you wish. Labels are useful if the program is to branch to that instruction or if the location contains data that is to be addressed explicitly.
The opcode field can be any one of the following instructions:
ADD, AND, BR, HALT, JMP, JSR, JSRR, LDB, LDW, LEA, NOP, NOT, RET, LSHF, RSHFL, RSHFA, RTI, STB, STW, TRAP, XORThe number of operands depends on the operation being performed. It can consist of register names, labels, or constants. If a hexadecimal constant is used, it must be prefixed with the 'x' character. Similarly, decimal constants must be prefixed with a '#' character.
Optionally, an instruction can be commented -- which is good style if the comment contains meaningful information. Comments follow the semicolon and are not interpreted by the Assembler. Note that the semicolon prefaces the comment, and a newline ends the comment. Other delimiters should not be used.
In this lab assignment, the NOP instruction translates into the machine language instruction 0x0000.
Note that you should also implement the HALT instruction as TRAP x25. Other TRAP commands (GETC, IN, OUT, PUTS) need not be recognized by your assembler for this assignment.
In addition to LC-3b instructions, an assembly language also contains pseudo-ops, sometimes called macro directives. These are messages from the programmer to the assembler that assist the assembler in performing the translation process. In the case of our LC-3b Assembly Language, we will only require three pseudo-ops to make our lives easier: .ORIG, .END, and .FILL.
An assembly language program will consist of some number of assembly language instructions, delimited by .ORIG and .END. The pseudo-op .END is a message to the assembler designating the end of the assembly language source program. The .ORIG pseudo-op provides two functions: it designates the start of the source program, and it specifies the location of the first instruction in the object module to be produced. For example, ".ORIG n" means "the next instruction will be assigned to location n." The pseudo-op ".FILL w" assigns the value w to the corresponding location in the object module. w is regarded as a word (16-bit value) by the .FILL pseudo-op.
The task of the assembler is that of line-by-line translation. The input is an assembly language file, and the output is an ISA file (consisting of hexadecimal digits). To make it a little more concrete, here is a sample assembly language program:
.ORIG x3000 ;This program counts from 10 to 0 LEA R0, TEN LDW R1, R0, #0 START ADD R1, R1, #-1 BRZ DONE BR START ;blank line DONE TRAP x25 ;The last executable instruction TEN .FILL x000A ;This is 10 in 2's comp, hexadecimal .END ;The pseudo-op, delimiting the source programAnd its corresponding ISA program:
0x3000 0xE005 0x6200 0x127F 0x0401 0x0FFD 0xF025 0x000ANote that each line of the output is a four digit hex number, prefixed with '0', representing the 16-bit machine instruction. If each instruction in the output is not prefixed with a '0', it will not be recognized by the simulator which you will write in Lab Assignment 2. Also note that BR instruction is assembled as the uncontional branch, BRnzp.
When this program is loaded into the simulator, the instruction 0xE005 will be loaded into memory location specified by the first line of the program which is 0x3000. As instructions consist of two bytes, the second instruction, 0x6200, will be loaded into memory location 0x3002. Thus, memory locations 0x3000 to 0x300C will contain the program.
We have included below another example of an assembly language program, and the result of the assembly process. In this case, we assume that the .ORIG pseudo-op allows assembly to start at an arbitrary memory address (e.g., 4096).
.ORIG #4096 A LEA R1, Y LDW R1, R1, #0 LDW R1, R1, #0 ADD R1, R1, R1 ADD R1, R1, x-10 ;x-10 is the negative of x10 BRN A HALT Y .FILL #263 .FILL #13 .FILL #6 .ENDWould be assembled into the following:
0x1000 0xE206 0x6240 0x6240 0x1241 0x1270 0x09FA 0xF025 0x0107 0x000D 0x0006Important note: Even though this program will assemble correctly, it may not do anything useful.
The second pass performs the translation from assembly language to machine language, one line at a time. It is during this pass that the output file should be generated.
You should write your program to take two command-line arguments. The first argument is the name of a file that contains a program written in LC-3b assembly language, which will be the input to your program. The second argument is the name of the file to which your program will write its output. In other words, this is the name of the file which will contain the LC-3b machine code corresponding to the input assembly language file. For example, we should be able to run your assembler with the following command-line input
where assemble is the name of the executable file corresponding to your compiled and linked program; <source.asm> is the input assembly language file, and <output.obj> is the output file that will contain the assembled code.
assemble <source.asm> <output.obj>
You will need to include some basic error checking within your assembler to handle improperly constructed assembly language programs. Your assembler must detect three types of errors and must return three types of error codes. The errors to be detected are undefined labels (error code 1), invalid instructions (error code 2), and invalid constants (error code 3). An invalid constant is a constant that is too large to be assembled into an LC-3b instruction. If the .ORIG pseudo-op contains an address that is greater than an address that can be represented in the 16-bit address space, your program should return error code 3. Also, if the .ORIG statement specifies an address that is not word-aligned, your program should return error code 3. Your program must return the error codes via the exit(n) function, where n denotes the error code number. If the assembly language program does not contain any errors, you must exit with error code 0.
This error checking is the bare minimum that we expect. You can return error code 4 for any other errors you find. Just be sure that the errors don't fall within the first three categories specified above.
To complete Lab Assignment 1, you will need to turn in the following:
Be sure that your assembler can handle comments on any line, including lines that contain pseudo-ops and lines that contain only comments. Be careful with comments that follow a HALT, NOP or RET instructions -- these instructions take no operand.
Your assembler should allow hexadecimal and decimal constants after both ISA instructions, like ADD, and after .FILL pseudo-ops.