cs-280 dr. mark l. hornick 1 storing data in program memory special-purpose load and store...

CS-280Dr. Mark L. Hornick

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Storing data in Program Memory

Special-purposeLoad and Store Instructions


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Atmega32 Memory

Address bus (16-bit in Atmega32) A unique 16-bit address

references each memory byte.

Data bus (8-bit)

Volatile – RAM (fast RW) SRAM (temp data store) DRAM

Nonvolatile – ROM (fast R – slow W) ROM PROM EPROM EEPROM (permanent data

store) Flash ROM (program store)


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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


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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

Byte 0

Byte 2

Byte 4

Byte 6

Byte 8

0x0000

0x0001

0x0002

0x0003

0x0004

Byte 1

Byte 3

Byte 5

Byte 7

Byte 9

Byte 0

Byte 1

Byte 2

Byte 3

Byte 4

0x0060

0x0061

0x0062

0x0063

0x0064


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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=0x60x2: .byte 2 ; reserve 2 bytes of SRAM, assign label x2=0x61x3: .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 x1STS 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


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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


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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


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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 instructionsy1: .db 1,5 ; alloc 2 bytes in Prog Memory with initial values of 1 and 5title: .db ‘c’,’s’,’2’,’8’,’0’,0 ; allocate 6 bytes in Program Memorycourse: .db “CS-280”, 0 ; allocate 7 bytes in Program MemoryNote 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


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Review: ASCII Encoding

For example, character 'O' is 79 (row value 70 + col value 9 = 79).

For example, character 'O' is 79 (row value 70 + col value 9 = 79).

O

9

70


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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


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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


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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 instructionsLDI ZL, LOW(2*values) ; load ZL with the low 8 bits of the addressLDI ZH, HIGH(2*values) ; load ZH with the high 8 bits of the addressLPM 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


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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 Memorycourse: .db “CS-280”, 0 ; allocate 7 bytes in Program Memory


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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


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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 1 storing data in program memory special-purpose load and store...

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