EET 2261 Unit 2 - PowerPoint Presentation

Slide1

EET 2261 Unit 2HCS12 Architecture

Read

Almy

,

Chapters 3, 4, and 5.

Homework #2

and Lab #2 due

next week.

Quiz next week.Slide2

Central Processing Unit (CPU)

For the next few weeks we’ll concentrate on the HCS12’s CPU,

which is described in the

CPU Reference Manual.Block diagram from page 6 of textbook or page 23 of Device User Guide.Slide3

CPU Programming Model

Inside any processor’s CPU are certain registers that the programmer has access to.

The list of these registers is often called the

CPU programming model. For the HCS12 (see also next slide):RegisterNumber of bitsAccumulator A8Accumulator B8

Accumulator D16

Index Register X

16

Index Register Y

16

Stack Pointer

16

Program Counter

16

Condition Code Register

8Slide4

HCS12 CPU Programming

Model

See page 30

of textbook or page 375 of CPU Reference Manual.CodeWarrior displays the values of these registers (plus two others) in its Register window.Slide5

Purposes of the Registers

Accumulators A, B, and D are general-purpose registers that the programmer can use however she wants.

PC and CCR are special-purpose registers that always perform specific functions:

PC always holds the address of the next instruction to be executed.CCR always holds three mask bits and five status flags.The other registers (X, Y, and SP) sometimes serve as general-purpose registers

and sometimes perform specific functions.Slide6

Accumulators A, B, and D

The HCS12 has two 8-bit accumulators, named A and B. These two can also be regarded as forming a single 16-bit accumulator named D.

Some instructions operate on Accumulator A or Accumulator B. Other instructions operate on Accumulator

D.Examples: LDAA loads an 8-bit number into A.LDAB loads an 8-bit number into B.LDD

loads a 16-bit number into

D.Slide7

Instruction Set

A processor’s

instruction set

is the list of instructions (or commands) that the CPU recognizes.Most of these instructions manipulate the CPU’s registers or locations in memory (or both).Slide8

HCS12 Instruction Set Summary

For a summary of the HCS12’s instruction set, see

the table on pages

381-394 of the CPU Reference Manual.Slide9

Explanation of Abbreviations in HCS12 Instruction Set Summary

This

Instruction Set Summary uses

many abbreviations and special symbols, which are explained on the pages before the table (pages 377-380 of the CPU Reference Manual.)Slide10

Example: ABA

Let’s look at one instruction, ABA, which adds Accumulator B to Accumulator A and puts the results in Accumulator A.

Note that this column has info about both our chip (HCS12) and an obsolete ancestor chip (M68HC12).Slide11

First Column: Source Form

This column shows how

you’ll type the instruction

when you start writing assembly language programs next week. ABA is very simple. Other instructions are more complicated and have additional things you need to type. Some abbreviations used in this column: Slide12

Second Column: Operation

This column explains

the operation that the

instruction performs.Some symbols used in this column:Slide13

Third Column: Addressing Mode

This column shows

the

addressingmode(s) used by the instruction. As we’ll see below, there aresix addressing modes.Some abbreviations used in this column:Slide14

Fourth Column: Machine Coding

Computers use numbers to

represent all kinds of

information, including the instructions in a program. This column shows the numbers (in hex) that represent each instruction.Some abbreviations used in this column:Slide15

Fifth Column: Access Detail

This column shows how

many cycles the

instruction takes to execute (two, for ABA), and what happens during each of those cycles.Some abbreviations used in this column:Slide16

Last Two Columns: Condition Codes

These columns show how

each instruction affects the bits

in the Condition Code Register.Symbols used in these columns:Slide17

Executing an ABA Instruction in CodeWarrior

Start a new CodeWarrior project, choosing Full Chip Simulation.

Start CodeWarrior’s Debugger.

Place an ABA instruction ($18 06) in memory locations $2000 and $2001.Place $2000 in the Program Counter so it points to our ABA instruction.Place some values (let’s say, 6 and 7) in Accumulators A and B.Single-step by pressing F11.Slide18

Review: Nibbles and Bytes

Recall that in a hex number such as $18 06:

Each hex digit represents a nibble (4 bits).

So each pair of hex digits represents a byte (8 bits).So the machine code for an ABA instruction takes up two bytes of memory. In our exercise, we put the first half of the instruction in address $2000, and the second half in address $2001.Some instructions take up only a byte of memory, but many take two, three, four, or more bytes.Slide19

Summary

This

table summarizes a

lot of useful information for every one of the 207 instructions that you can use when writing programs for the HCS12.You’ll refer to this table often, so it might be a good idea to print these 14 pages (pp. 381-394 in the CPU Reference Manual).Slide20

Instruction Set Details

For more detailed info on all 207 of

the HCS12’s

instructions, see pages 100-309 of the CPU Reference Manual.Slide21

RISC Architecture versus CISC

Architecture

Some commercially available microprocessors use a RISC (Reduced Instruction Set Computer) architecture.

Small instruction set (maybe 25 or 30 instructions).Each instruction performs a simple operation and executes quickly.The HCS12 is an example of a CISC (Complex Instruction Set Computer) processor.Large instruction set (207 in the HCS12’s case).

Some instructions perform complex operations that might require a dozen or more instructions in a RISC processor.Slide22

Categories of Instructions

The Instruction Set Summary table lists instructions alphabetically. Sometimes it’s more useful to have instructions grouped into categories of similar instructions.

Examples of categories:

Load and Store InstructionsAddition and Subtraction InstructionsBoolean Logic Instructions…See

Section 5 (starting on p. 55) of the CPU Reference Manual.Slide23

Mnemonics

In this course we’ll write programs in a language called

assembly language

.Assembly language contains many mnemonics, which are abbreviations for actions that we want to perform. (Mnemonics appear in the Source Form column of the Instruction Set Summary.)Some examples:Mnemonic

Action

ABA

Add Accumulator B to Accumulator

A

LDAA

Load a number into Accumulator A

STAA

Store Accumulator A into memorySlide24

Operands

Some HCS12 instructions require the programmer to specify one or two

operands

, in addition to the mnemonic.These operands are the data (usually numbers) to be operated on.Example: The LDAA instruction loads a number into accumulator A. The number that’s to be loaded is the operand.In the instruction LDAA #5, the operand is 5.Slide25

Opcodes

When an instruction is translated into machine code, the mnemonic is replaced by an

opcode

.Example #1: The assembly-language instruction ABAbecomes $18 06

in machine code, because $18 06

is the opcode for

ABA.

Example #2: The assembly-language instruction

LDAA #5

becomes

$86 05

in machine code, because $86 is the opcode for LDAA.

Mnemonic

Operand

Opcode

Mnemonic

OpcodeSlide26

Review: Memory Addresses

Every computer system has memory, where programs and data are stored.

Each location within this memory has an

address by which we identify it. And each location holds some contents, which is one byte.The HCS12 uses a 16-bit address bus.So the addresses of its memory locations range from $0000 to $FFFF. (In decimal, that’s 0 to 65,535.) So we have 65,536 memory locations, each of which holds one byte.Slide27

Review: Memory Map

Programmers must pay attention to where in memory their programs and data are stored.

Some of a system’s memory is reserved for special purposes. Therefore, within the range of possible addresses, only some can be used by your programs.

A memory map shows which memory addresses are reserved, and which are available for your use.Next slide shows memory map for our HCS12 chip.Slide28

See page 26 of

Device User Guide

.

Reserved addresses:$0000 to $03FF are reserved for special-function registers.$FF00 to $FFFF are reserved for interrupt vectors.HCS12 Memory MapSlide29

Addressing Modes

Every instance of an instruction uses an

addressing mode

.The HCS12’s six addressing modes are:Inherent (abbreviated INH)Immediate (IMM)Direct (DIR)Extended (EXT)Indexed (IDX), which has several variations

Relative (REL)Slide30

Which Addressing Mode(s) Does an Instruction Use?

Some instructions come in only one “flavor”: they can only use one addressing mode.

Example: The ABA instruction

always uses the inherent addressing mode.Other instructions come in two or more “flavors.”Example: The LDAA instruction can use immediate, direct, extended, or indexed addressing.Recall that the Instruction Set Summary lists the possible addressing modes for every instruction.Slide31

Addressing

Mode #1:

Inherent Addressing Mode

In inherent addressing mode, the programmer doesn’t specify any operands, because the mnemonic itself contains all of the information needed.Example: The ABA instruction adds the numbers in Accumulators A and B. So we don’t need to say anything more than ABA. We just type ABA, instead of typing something like ABA #5.

This is incorrect. Typing it in a programwould be an error and the program would

not run.Slide32

Addressing Mode #1:

Inherent Addressing

Mode

(Cont’d.)Recall that when an instruction is translated from assembly language into machine code, the mnemonic is replaced by an opcode.The Instruction Set Summary tells us that the opcode for ABA is $18 06, so the assembly-language instruction ABA

becomes

$18 06

in

machine

code.

Mnemonic

OpcodeSlide33

Addressing Mode

#2:

Immediate Addressing Mode

In immediate addressing mode, the programmer gives the operand(s) as part of the instruction. The pound sign (#) indicates immediate addressing.Example: LDAA #15 causes the number 15 to be loaded into Accumulator A.You’ll get an error if you just type LDAA and try to run your program.

The following are all equivalent:

LDAA #15

LDAA #$F

LDAA #%1111Slide34

Addressing Mode #2:

Immediate Addressing

Mode

(Cont’d.)The Instruction Set Summary tells us that the opcode for an immediate-mode LDAA is $86, so the assembly-language instruction LDAA #$Fbecomes

$86 0F

in

machine

code.

Mnemonic

Opcode

OperandSlide35

Addressing Mode

#3:

Direct Addressing Mode

In direct addressing mode, the programmer gives the operand’s address as part of the instruction. Example: LDAA $33 causes the number located at memory address $33 to be loaded into Accumulator A.The following are all equivalent:LDAA 51LDAA $33

LDAA %00110011Slide36

Addressing Mode #3:

Direct Addressing Mode

(Cont’d.)The Instruction Set Summary tells us that the opcode for a direct-mode LDAA is $96, so the assembly-language instruction LDAA $33becomes

$96 33 in

machine

code.

Mnemonic

Opcode

Operand’s addressSlide37

Limitation of Direct Addressing Mode

In direct addressing mode, the operand’s address can only be one byte long.

So the highest address we can use with direct addressing is $FF, or 255.

Since there are 65,536 locations in memory, this means that 65,281 of those locations can’t be reached by direct addressing!Solution: use extended addressing mode instead of direct addressing mode. (See next slide.)Slide38

Addressing Mode

#4:

Extended Addressing Mode

In extended addressing, the programmer gives the operand’s address as part of the instruction (just as in direct addressing). But this address is two bytes long, so we can reach any memory location.Example: LDAA $701F causes the number located at memory address $701F to be loaded into Accumulator A.The following are all equivalent:LDAA 28703LDAA $701F

LDAA %0111000000011111Slide39

Addressing Mode #4:

Extended Addressing Mode

(Cont’d.)The Instruction Set Summary tells us that the opcode for an extended-mode LDAA is $B6, so the assembly-language instruction LDAA $701Fbecomes

$B6 70 1F

in

machine

code.

Mnemonic

Opcode

Operand’s addressSlide40

Summary: Addressing Modes

We’ve discussed the first four of the HCS12’s six addressing modes:

Inherent

(Tipoff: just a mnemonic, with no number.)Immediate (Tipoff: # between the mnemonic and number.)Direct (Tipoff: Mnemonic followed by a 1-byte number, no #.)

Extended

(Tipoff: Mnemonic followed by a

2-byte

number, no #.)

Indexed

Relative

We’ll discuss the other two modes in the weeks ahead.Slide41

Condition Code Register

The Condition Code Register (CCR) is an 8-bit register that contains:

5 flag bits (H, N, Z, V, C)

2 interrupt mask bits (X and I)STOP instruction control bit (S)Slide42

Meanings of the H, Z, and C Flag Bits

The flag bits are set or reset based on the results of previously executed instructions.

H

=1 if a half-carry (carry from bit 3 to bit 4) occurred.Z=1 if the result was zero.C=1 if a carry out of the leftmost bit occurred (or, in the case of subtract instructions, if a borrow into the leftmost bit occurred

).

But

only some instructions affect these flag bits

: check the Instruction Set Summary table.Slide43

Review: Two’s-Complement Notation

Recall that

two’s-complement notation

is the standard way of representing signed numbers in binary. In this notation, the leftmost bit is the sign bit.Sign bit = 0 for positive numbers or 0.Sign bit = 1 for negative numbers.Examples, using 8 bits:%0000 0011 = 3

%1000 0011 = 125Slide44

Meanings of the N and V Flag Bits

N

=1 if the result’s leftmost bit (MSB) was 1. If you’re using signed numbers, this means that the result was

negative.V=1 if a two’s-complement overflow occurred. This happens when either:We added two positive numbers (MSB=0) and got a negative result (MSB=1).Or we added two negative numbers (MSB=1) and got a positive result (MSB=0).Again,

only some instructions affect these flag bits

: check the

Instruction Set Summary

table.