Storing data in Program
Memory
Special-purpose
Load and Store Instructions
CS-280
Dr. Mark L. Hornick
1
Atmega32 Memory

Address bus (16-bit in
Atmega32)




Volatile – RAM (fast RW)


SRAM (temp data store)
DRAM
CS-280
Dr. Mark L. Hornick
A unique 16-bit address
references each memory
byte.
Data bus (8-bit)
Nonvolatile – ROM (fast R – slow W)





ROM
PROM
EPROM
EEPROM (permanent data store)
2
Flash ROM (program store)
SRAM is volatile memory



Power must be supplied to maintain values
stored in SRAM
SRAM values disappear when power is shut
off
SRAM is cleared when power is first applied
CS-280
Dr. Mark L. Hornick
3
Review: Program vs. Data
Memory Addressing

Program Memory is
organized and accessed in
words



A word is 2 bytes
Each word has a unique
address
Data Memory is organized
and accessed in bytes

Each byte has a unique
address
CS-280
Dr. Mark L. Hornick
0x0000
Byte 0
Byte 1
0x0001
Byte 2
Byte 3
0x0002
Byte 4
Byte 5
0x0003
Byte 6
Byte 7
0x0004
Byte 8
Byte 9
0x0060
Byte 0
0x0061
Byte 1
0x0062
Byte 2
0x0063
Byte 3
0x0064
Byte 4
4
Review: Allocating SRAM Data
memory
.DSEG
; subsequent directives refer to data segment
.ORG SRAM_START ; address where Data Memory starts (0x60)
x1:
.byte 1 ; reserve 1 byte of SRAM, assign label x1=0x60
x2:
.byte 2 ; reserve 2 bytes of SRAM, assign label x2=0x61
x3:
.byte 1 ; reserve 1 byte of SRAM, assign label x3=0x63
.CSEG
; switch further directives to code segment
.ORG 0x2A
; set addr for start of program instructions
LDS R20, x1 ; load value at data addr specified by x1
STS x2, R20 ; store value in R20 to data addr x2

The addresses are assigned automatically by the Assembler
 The .byte n directive tells the assembler to allocation n bytes
 Be very careful about using .ORG for addresses < 0x60
CS-280
Dr. Mark L. Hornick
5
Review: The X, Y, and Z
Registers
The X, Y, and Z 16-bit registers
overlap the last six 8-bit registers R26
through R31
CS-280
Dr. Mark L. Hornick
6
Review: Indirect Addressing

Accesses a 8-bit (byte) value from SRAM
data memory at the address specified indirectly via
the 16-bit X, Y, or Z index-registers

Example:



LD R20, X
; load value at data addr held in X to R20
ST Y, R20
; store value in R20 to data addr held in Y
Note X, Y, Z hold data address values that are 16-bits
CS-280
Dr. Mark L. Hornick
7
Data values can be stored permanently
by allocating them to non-volatile
Program Memory
.DSEG
; subsequent directives refer to data segment
.ORG
SRAM_START ; address where Data Memory starts (0x60)
x1: .byte 5
; reserve 5 bytes of (uninitialized) volatile SRAM
.CSEG
; switch further directives to code segment
.ORG
0x2A
; set addr for start of program instructions
y1:
.db 1,5
; alloc 2 bytes in Prog Memory with initial values of 1 and 5
title:
.db ‘c’,’s’,’2’,’8’,’0’,0
; allocate 6 bytes in Program Memory
course: .db “CS-280”, 0
; allocate 7 bytes in Program Memory
Note assembler does NOT automatically insert a NULL char at the end of the string

The .byte n directive tells the assembler to allocate n bytes in Data Memory


No initial value can be specified to SRAM data memory values
The .db n,m,… (“define byte”) directive tells the assembler to allocate and
store the bytes n,m… in Program Memory

The initial values of the memory are specified

The assembler always starts data on a word boundary in memory
CS-280
Dr. Mark L. Hornick
8
Review: ASCII Encoding
9
O
70
CS-280
Dr. Mark L. Hornick
For example,
character 'O' is
79 (row value
70 + col value 9
= 79).
9
Accessing (reading) data from
Program Memory
There are three instructions to load (read) data
from Program Memory to a register
LPM

loads from byte address in Z register to R0,
where Z and R0 are implied arguments
LPM Rn, Z

loads from byte address in Z to Rn (R0-R31); note
that X and Y are not allowed
LPM Rn, Z+

Same as above, but increments Z after loading
CS-280
Dr. Mark L. Hornick
10
Big Red Flag


When using LPM, the Z register must contain
the data’s byte address
Recall: Program Memory is organized by
words – each word has an address

Byte address is 2*word address
CS-280
Dr. Mark L. Hornick
11
How do you load a byte address
value into Z?
.CSEG
.ORG
LDI
LDI
LPM
; switch further directives to code segment
0x2A
; set addr for start of program instructions
ZL, LOW(2*values) ; load ZL with the low 8 bits of the address
ZH, HIGH(2*values) ; load ZH with the high 8 bits of the address
R20, Z
; load Program Memory byte addr contained in Z
values: .db 5, 10, 11, 12, 13

; define 5 bytes of Program Memory
Use LOW() and HIGH() are Assembler functions
 “values” label contains the word address of the
location of the first reserved byte
 The assembler expression “2*values” correctly
computes the associated byte address
CS-280
Dr. Mark L. Hornick
12
There are certain conventions for
defining Program Memory

The .db directives are placed after program instructions, so that the
data itself is placed in memory after the instruction opcodes

When defining numeric data values, the first value specifies the
number of values to follow:
y1:

.db 1,5 ; alloc 2 bytes in Prog Memory with initial values of 1 and 5
When defining character or string data values, the last value
speicified should always be zero, indicating that the character data
is terminated with a null-character:
title:
course:
.db
.db
‘c’,’s’,’2’,’8’,’0’,0
“CS-280”, 0
; allocate 6 bytes in Program Memory
; allocate 7 bytes in Program Memory
CS-280
Dr. Mark L. Hornick
13
Note: The Z register can also be
incremented explicitly

ADIW ZH:ZL, 5 ; add immediate value (to Z)


The value may be 0-63 (decimal)
So you can use ADIW in conjuction with LPM
and LPM Rn, Z instructions to post-increment
the Z register to point to the address of the next
character in program memory
CS-280
Dr. Mark L. Hornick
14
There is also an SPM
instruction….

But due to the nature of Flash memory, writing to it is
more complex




Before writing, an entire page of memory must be erased
Page size is device-specific
On some devices, only entire pages can be written
In CE2800, use the .DB directive to write to Flash
memory

Only writes once, when the program is downloaded to the
device
CS-280
Dr. Mark L. Hornick
15