CSC270   HW #5

Problem #1

Returning to the traffic light controller from HW 4, assume now that we want the same behavior of the traffic light circuit of Problem 1, HW 4 (3 seconds green, 2 seconds yellow, 2 seconds red), but this time the clock signal available has a frequency of 4 Hz.

Note that you can do this by retaining the circuit from HW 4 and simply adding a new stage to create the desired clock signal.

Show your new diagram, and highlight the parts in it that are taken directly from your answer to the first problem.

Problem #2

Write a 6800 assembly language program that converts all the characters of and ASCII string to uppercase. (You can assume that the string is zero-terminated, providing you with something to check for your stop condition.) Assume that the string is stored in a variable located at address 0000, and is at most 80 chars in length.

Your answer should contain a fully-documented program, with the instructions and their assembled op-codes in hexadecimal.

Problem #3

Reconstruct the program whose hexadecimal dump is shown below. This program was located starting at Address 0000 in RAM, and started execution from this address.

7E 00 0A 00 00 00 00 00 
00 00 CE 00 03 C6 05 86 
01 97 00 9B 00 08 5A 26 
F8 7E FC 00

Your reconstructed program should contain a header explaining the general behavior of the program, the assembly language instructions and their equivalent hex opcodes, each with a comment explaining how each instruction contributes to the general computation.

Problem #4

This is the same question as for Problem #3, but this time with a more challenging sequence. Assume here as well that the execution starts at 0000.

7E 00 07 03 7E 00 07 86
00 97 06 C6 FC D7 05 CE
03 7E DF 03 20 EE

Your answer should contain a clear explanation of the behavior of the program.

Problem #5: opcode bit-fields

Observe the difference between an LDAA direct and an LDAB direct: one is 96 hex, the other D6 hex. Or 10010110 compared to 11010110. The difference is just one bit: the second from the left. This means that very likely the second bit from the left indicates that the register selected is ACCA if 0, and ACCB if 1.

Look a the op-codes supported by the 6800 and attempt to figure out how the 6800 decomposes the opcode to understand what it has to do and how it has to do it. In particular, answer the following questions:

1. What bit or bits define the addressing mode (direct, immediate, etc.)?
2. What bit or bits define the operation (add, sub, and, or, etc.)?
3. Are there bits that identify the type of the instruction? In other words, are instructions using the Accumulators (e.g. LDAA) different in their bit patterns from the instructions that work on the index register (e.g. LDX), or from the branch instrutions, or from other instructions?
4. Are there bits that have different meanings depending on how other bits are set? For example, does bit j mean something if bit i is 0, and means something else if bit i is 1?