21 April Lab in CSCI 274

Lab goals

Starting off

The file c5asm is a shell script that invokes an assembler, written by Wayne Lang, for the Chapter 5 computer. The assembler is a tad rough in places. Consider it a work-in-progress or a good opportunity for a senior project. A Veriwell RTL specification, also written by Wayne Lang, suitable for executing the output of the assembler is found in C5rtl.v. The final file, loop.asm, is a sample assembler program.

Remember! The veriwell simulator will only run on a Sun SPARC workstation. If you are not logged into one of these machines type the following commands:

Examining the program

Let's start by looking at loop.asm, the sample assembler program. The Verilog program that simulates the Chapter 5 computer always starts executing at memory location 0. Consequently our program starts with a branch to PRG, the read start of the code

    ORG 000

The program itself is a single loop, with a bit of initialization code. Ten numbers are stored beginning at the memory location called VEC. The program adds up these ten numbers and stores the result in SUM.

The first step of the program is to initialize SUM to 0, initialize VPT to point to the first element of VEC, and to initialize TOX to -10.


The loop proceeds by increment VPT to point to successive elements of VEC. The up-counter TOX goes from -10 to 0 to make sure the loop is only executed 10 times.


Compiling the program

Run Wayne Lang's assembler by typing

You'll see some informative messages flash by. Now use pico to examine the file loop.out that was created in your directory. The file will begin with a "label table" that will give the locations of your programs address in memory. Lines of the table will resemble

which tells us that the variable SUM is stored at memory location 20A. The remainder of the file is a assembler listing. It tells us the machine instructions associated with each assembler statement.

The c5asm program also created a file loop.bin. This file is not intended for Veriwell input not for human consumption; however, you should still look at loop.bin and relate it to your assembler program.

Running the program

The Veriwell program C5rtl.v is a faithful, but incomplete, specification of the RTL found on page 159 of the textbook. Let's not worry about the details of C5rtl.v this week. Instead just run Veriwell to simulate the loop program by typing

Look at the file veriwell.log, the debugging output of the simulation. Wayne Lang's Veriwell program outputs a line similar to

whenever a machine language instruction is simulated. Only the AC= field and the last field, the current instruction, interests us at this time. Now go through the file veriwell.log and trace through the execution of your program.

Your job

Modify the loop.asm program either to compute the exclusive-OR (XOR) of the ten array values or to add only the positive elements found within the ten array values.

If you compute the XOR of the ten values, you'll need to compute the exclusive OR with only the NOT and AND operators. This can be done as

By the way, it is probably easier to add up the positive elements.

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