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Designer’s

Handbook 13

to

flash

memory.

It

may

be

mapped

to

RAM

for

debugging,

or

if

it

is

decided

to

copy

code

to

RAM

to

take

advantage

of

faster

access

time

offered

by

RAM.

The

root

segment

holds

a

mixture

of

code

and

constants.

C

functions

or

assembly

language

programs

that

are

com-

piled

to

the

root

segment

are

interspersed

with

data

constants.

Data

constants

are

inserted

between

blocks

of

code.

Data

constants

defined

inside

a

C

function

are

emitted

after

the

end

of

the

code

belonging

to

the

function.

Data

constants

defined

outside

of

C

functions

are

emitted

as

encountered

in

the

source

code.

Except

in

small

programs,

the

bulk

of

the

code

is

executed

using

the

extended

memory

window.

But

the

root

segment

has

special

properties

that

make

it

better

for

some

types

of

code.

The

types

of

subroutines

and

functions

that

are

best

placed

in

the

root

segment

are

as

follows.

1. Short

subroutines

of

about

20

instructions

or

less

that

are

frequently

called

will

use

sig-

nificantly

less

execution

time

if

placed

in

the

root

because

of

the

faster

calling

linkage

for

16-bit

versus

20-bit

addresses.

A

call

and

return

using

16-bit

addressing

requires

20

clocks,

compared

to

32

clocks

for

20-bit

addressing.

2. Interrupt

routines.

Interrupts

use

16-bit

addressing

so

the

entry

to

an

interrupt

routine

must

be

in

root.

3. Functions

called

indirectly

using

traditional

C

pointers.

4. The

BIOS

core.

A

certain

part

of

the

BIOS

must

be

at

the

start

of

the

root

segment.

Figure

3.

Typical

Layout

of

Root

Segment

4.1.2

The

Data

Segment

The

data

segment

is

mapped

to

RAM

and

contains

C

variables.

Typically

it

starts

at

8K

or

above

and

ends

at

52K.

Data

allocation

starts

at

or

near

the

top

and

proceeds

in

a

down-

ward

direction.

It

is

also

possible

to

place

executable

code

in

the

data

segment

if

it

is

cop-

ied

from

flash

to

the

data

segment.

This

can

be

desirable

for

code

that

is

self

modifying,

code

to

implement

debugging

aids

or

code

that

controls

write

to

the

flash

memory

and

0K

System

Constants

BIOS

Start

System

ID

(256

bytes)

BIOS

Functions

Constant

defs

Max

48K

Table of Contents

Other manuals for Rabbit 2000

User Manual230 pages

Instruction & Reference Manual174 pages

Reference Manual120 pages

Getting Started Manual54 pages

Getting Started43 pages