Computer Organization and Assembly Language Programming

Assignment

1- Consider the internal structure of the pseudo-CPU discussed in class augmented with a single-port register file (i.e., only one register value can be read in a cycle) containing 32 8-bit registers (R0-R31) and a carry bit (C bit), which is set/reset after each arithmetic operation.

Suppose the pseudo-CPU can be used to implement the AVR instruction ADIW ZH:ZL,32 (Add immediate to word). ADIW is a 16-bit instruction, where the upper byte represents the opcode and the lower byte represents an immediate value, i.e., "32" (do not worry about the fact that the actual format is slightly different).

Give the sequence of microoperations required to Fetch and Execute the ADIW instruction. Your solutions should result in exactly 5 cycles for the fetch cycle and 6 cycles for the execute cycle.

Assume the memory is organized into addressable bytes (i.e., each memory word is a byte), MDR, IR, and AC registers are 8-bit wide, and PC and MAR registers are 16-bit wide. Also, assume Internal Data Bus is 16-bit wide and thus can handle 8-bit or 16-bit (as well as portion of 8-bit or 16- bit) transfers in one microoperation and only PC and AC have the capability to increment itself.

2- Consider the internal structure of the pseudo-CPU discussed in class augmented with a single-port register file (i.e., only one register value can be read at a time) containing 32 8-bit registers (R31-R0) and a Stack Pointer (SP). Suppose the pseudo-CPU can be used to implement the AVR instruction ICALL (Indirect Call to Subroutine) with the format shown below:

ICALL pushes the return address onto the stack and jumps to the 16-bit target address contained in the Z register. Give the sequence of microoperations required to Fetch and Execute AVR's ICALL instruction. Your solutions should result in exactly 6 cycles for the fetch cycle and 8 cycles for the execute cycle.

Assume the memory is organized into addressable bytes (i.e., each memory word is a byte), MDR is 8 bits, and AC, SP, PC, IR, and MAR are 16 bits. Also, assume Internal Data Bus is 16-bit wide and thus can handle 8-bit or 16-bit (as well as portion of 8-bit or 16-bit) transfers in one microoperation and SP has the capability to increment/decrement itself. Clearly state any other assumptions made.

3- Suppose the following array of numbers are stored in the Data Memory (represented in hexadecimal):

(a) Assuming these numbers are signed numbers, write a subroutine using AVR assembly that (1) determines the smallest number among the 8 numbers stored in memory and (2) stores that number in the memory location $0108. Clearly comment and explain your code. Use the skeleton code shown below to implement your subroutine:

.ORG 0x0046
...Initialize stack...
RCALL MIN
......
.ORG 0x0060
MIN:
... ; Your code goes here
... ;
RET
.DSEG
.ORG 0x0100
DATA: .BYTE 8
RESULT: .BYTE 1

(b) Suppose these numbers are unsigned numbers (i.e., they are positive numbers). Show and explain how the code developed in part (a) would have to be modified.

4- Determine the location (i.e., address) and binary code for each instruction in the code developed for Problem #3 part (a). Clearly explain your answers. Examples of addresses and machines codes for RCALL and RET are shown below:

.ORG 0x0046
RCALL MIN
...
...
.ORG 0x0060
MIN: ...
...
RET

Address Binary

0046: 1101 0000 0001 1001
...
...
????: 1001 0101 0000 1000