CDAC 3331 Intro to Microprocessor Systems – Lecture 3
Dr. Michael VanHilst
Today we will learn about how the 68000 views memory, and about the syntax of the
assembly language for the 68000. This material is all covered in Chapter 2 of the
textbook. Here is an outline of the material for today.
Data Types
Memory model
Big-endian vs. Little-endian
Memory maps
Compile-Link-Load vs. Assemble-Load
Source code notations and format
Types of Instructions
Sizes of data:
The 68000 can operate on data in sizes of 32 bits, 16 bits, 8 bits and 1 bit.
A 32 bit value is called a long or long-word. 32 bits can represent 2^32 different
values, or roughly 4 billion. In the assembly language, long-word operations have
a .L suffix.
A 16 bit value is called a word. 16 bits can represent 2^16 different values, or
64K. The actual limits for unsigned 16 bit values are 0 to 65,535. For signed
values, the limits are -32,768 to 32,767. In the assembly language, word
operations have a .W suffix.
An 8 bit value is called a byte. 8 bits can represent 2^8 different values or 256.
The range of values goes from 0 to 255. In the
A 1 bit value is simply called a bit. Within a byte, word, or long-word, each bit
has an index. On the 68000, the bit in the one’s place, or the low order bit, is bit
0. The high bit in a byte, is bit number 8, in a word is bit number 15, and in a
long is bit number 31.
A note on binary values:
2^0 = 1
2^5 = 32
2^10 = 1K (1024)
2^15 = 32K (32768)
2^20 = 1M (1,048,576)
2^25 = 32M (33,554,432)
2^30 = 1G (1,073,741,824)
2^4 = 16
2^8 = 256
2^16 = 64K (65536)
2^24 = 16M (16,777,216)
2^32 = 4G (4,294,967,296)
The table above shows different binary numbers together with their short hand and actual
decimal equivalents. The column on the left shows a pattern that provides an easy way to
remember the powers of 2. Note that 2 to multiples of 10 correspond to the powers of 10
that we normally use for counting (1K, 1M, 1G). The in-between values of 2 raised to a
multiple of 5 are always 32 times the previous 2 raised to a multiple of 10. Thus the
