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Notes
This (complete) lab exercise should be printed and stapled before coming to lab. All
questions requiring a written answer should be answered by writing in the spaces
provided in this lab document, and the document should be submitted for marking.
Remember to keep this document for study purposes. Also, you will need the signed
version of this document if a re-grade or grade correction is requested.
1
Bit-Level Operations and Introduction to Stack
Objectives
The purpose of this lab is twofold: (1) to look at bit-level operations that process individual bits
of a byte, word, or long-word, and (2) to take a first look at how runtime stacks are implemented
on the 68000.
Upon completion of this lab students will be able to:
• Use the basic shift instructions to perform fast bit-level tasks, and
• Explain why processors typically support separate stacks for user and system processes.
During this lab you will make use of the following 68000 assembly-language programs:
• L7_1.x68
• L7_2.x68
To be written by you
To be written by you
Equipment:
In this lab you can either develop the L7_1.x68 on the 68KMB or on the Easy68K emulator. If
you chose to use the Easy68K, please note that (1) no official class support is provided for this
tool, and (2) you must still demonstrate your code running correctly on the 68KMB. With
regards to the latter, please remember that the TRAP instructions on the Easy 68K are
implemented differently than on the 68KMB. Also, the 68KMB only has 8K words of RAM
starting at hexadecimal address $8000. The second program, L7_2.x68, must be developed on
the 68KMB.
Part A: Programming Exercise
The “Russian Peasant” method of multiplication computes the product of two unsigned numbers
by doubling/halving of the factors. The algorithm is described through the example of
multiplying 30 x 25. First, put the factors in two columns. On each iteration double the value in
column 1 and divide the value in column 2 by 2. The process terminates when the quotient
reaches 1. The product is formed by adding the terms in column 1 whose corresponding term in
column 2 is odd.
Column1
30
60
120
240
480
Column2
25
Add 30
12
6
3
Add 240
1
Add 480
The product of 30 x 25 = 30 + 240 + 480 = 750.
2
(Apparently, real Russian peasants may have tracked their doublings with bowls of pebbles
instead of columns of numbers. The ancient Egyptians also invented a similar process thousands
of years earlier, and computers are still using related methods today.)
Your task is to write a program that implements the Russian Peasant algorithm to perform
unsigned multiplication. Assume the two factors (numbers to be multiplied) are 16-bit values
stored in data registers D0 and D1, respectively. When implementing your algorithm do not use
the 68000’s multiplication or division instructions. Rather, use the (faster) shift instructions
when performing multiplication and division by 2.
Save your program in
H:\2030\L7\L7_1.x68. [10 marks]
Part B: Hardware Stacks
Review your class notes, handouts on stacks and tracing, and the appropriate stack-related
instructions found in Chapter 3 of your textbook before proceeding.
To gain a better understanding of the 68000’s two runtime stacks, create the following program
(L7_2.x68) and assemble it.
RESULT
START
GENMUL
ORG
DC.L
$9000
0
ORG
ANDI.W
LEA
JSR
TRAP
$9500
#%1101111111111111,SR
$8100,A7
GENMUL
#14
MOVE.L
MOVE.L
MULS
MOVE.L
MOVE.L
RTS
END
D0,-(A7)
D1,D0
D2,D0
D0,RESULT
(A7)+,D0
START
Figure 1: Program that uses the 68KMB’s user stack (L7_2.x68).
Read the previous program carefully, and understand what it does. Then answer the following
questions pertaining to the program.
3
1. What does the first instruction do? ANDI.W
#%1101111111111111,SR [1 mark]
Answer:
Before running the program on the 68KMB you should press the reset button to ensure that the
board is in supervisor mode (i.e., the S-bit = 1).
2. Execute the program according to the instructions that follow:
a) Load D0, D1, and D2 with $12345678, $9ABCDEF0, and $0FEDCBA9,
respectively.
b) Execute the program up to (but not including) the JSR instruction. What are the
contents of the US register? What is this value referring to? [1 mark]
Answer:
c) Execute the JSR instruction with a trace command. What instruction is to be executed
next? What does it do? [1 mark]
Answer:
d) What are the contents of A7, US, SS immediately after the JSR instruction executes?
Why is A7 equal to SS? [1 mark]
Answer:
e) What do the contents of these registers signify? [1 mark]
Answer:
f) Execute the next instruction. What are the contents of US and why? [1 mark]
Answer:
g) Display the contents of the 8 bytes at the top of the user stack. What do these bytes
represent? [1 mark]
Answer:
4
h) Display the contents of the 8 bytes at the top of the system stack. What do these bytes
represent? [1 mark]
Answer:
i) Execute the next three instructions. What result is in D0? [1 mark]
Answer:
j) Execute the next instruction. What gets loaded in D0 and what is the value contained
in the US register? [1 mark]
Answer:
k) Run the program to completion with a GO command. What are the contents of the D0,
D1, D2, and US registers afterwards? [1 mark]
Answer:
l) What value gets stored at RESULT? [1 mark]
Answer:
Having completed this lab you should have a basic understanding of the operation of shift
instructions and how they can be used, as well as a basic understanding of the two runtime
stacks (user and system) employed by the computer. Next time, we will see how the C
programming language uses these stacks to implement function calls.
5