1. Storing data in Program Memory
Special-purpose Load and Store Instructions
CS-280
Dr. Mark L. Hornick

2. Atmega32 Memory Address bus (16-bit in Atmega32) Data bus (8-bit)
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)
Flash ROM (program store)
Volatile – RAM (fast RW)
SRAM (temp data store)
DRAM
CS-280
Dr. Mark L. Hornick

3. 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

4. Review: Program vs. Data Memory Addressing
0x0000
0x0001
0x0002
0x0003
0x0004
Byte 0
Byte 1
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
Byte 2
Byte 3
Byte 4
Byte 5
Byte 6
Byte 7
Byte 8
Byte 9
0x0060
0x0061
0x0062
0x0063
0x0064
Byte 0
Byte 1
Byte 2
Byte 3
Byte 4
CS-280
Dr. Mark L. Hornick

5. Review: Allocating SRAM Data memory
.DSEG ; subsequent directives refer to data segment
.ORG SRAM_START ; address where Data Memory starts (0x60)
x1: .byte ; reserve 1 byte of SRAM, assign label x1=0x60
x2: .byte ; reserve 2 bytes of SRAM, assign label x2=0x61
x3: .byte ; 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

6. 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

7. 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

8. 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 ; 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, ; 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

9. Review: ASCII Encoding
CS-1020
4/21/2017
Review: ASCII Encoding
For example, character 'O' is 79 (row value 70 + col value 9 = 79). O 9 70
Characters can be stored in a computer memory using the ASCII encoding. The ASCII codes range from 0 to 127. The character 'A' is represented as 65, for example. The ASCII values from 0 to 32 are called nonprintable control characters. For example, ASCII code 04 eot stands for End of Transmission. We can use this character to signal the end of transmission of data when sending data over a communication line.
CS-280
Dr. Mark L. Hornick
Dr. Mark L. Hornick

10. 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

11. 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

12. How do you load a byte address value into Z?
.CSEG ; switch further directives to code segment
.ORG 0x2A ; set addr for start of program instructions
LDI ZL, LOW(2*values) ; load ZL with the low 8 bits of the address
LDI ZH, HIGH(2*values) ; load ZH with the high 8 bits of the address
LPM 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

13. 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: db ‘c’,’s’,’2’,’8’,’0’,0 ; allocate 6 bytes in Program Memory
course: .db “CS-280”, 0 ; allocate 7 bytes in Program Memory
CS-280
Dr. Mark L. Hornick

14. 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

15. 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