accessing memory chapter 5

Accessing MemoryChapter 5

Lecture notes for SPARC Architecture, Assembly Language

Programming and C, Richard P. Paul

by Anu G. Bourgeois

2

Memory

• Addresses are 32 bits wide

• Therefore, 232 bytes in memory

• Each location is numbered consecutively

• Memory data types– Byte = 1 byte Halfword = 2 bytes– Word = 4 bytes Doubleword = 8 bytes

3

Data types

C Type SPARC Bits Unsigned Signed

char byte 8 0, 255 -128, 127

short half 16 0, 65,535 -32,768, 32,767

int, long word 32 0, 4.294x109 2.147 x 109

• All memory references must be aligned• x byte quantities must begin in an address divisible by x

Memory Allocation

.section “.data”input: .word 0x12345678limit: .half 0x9aprompt: .asciz “Hello!”

Address (decimal)

Data(hex)

299

300 78

301 56

302 34

303 12

304 9a

305 00

306 ‘H’

307 ‘e’

308 ‘l’

309 ‘l’

310 ‘o’

311 ‘!’

312

3134

input

limit

prompt

Memory Allocation

.section “.data”input: .word 0x12345678limit: .half 0x9aprompt: .asciz “Hello!”

Address (decimal)

Data(hex)

299

300 78

301 56

302 34

303 12

304 9a

305 00

306 ‘H’

307 ‘e’

308 ‘l’

309 ‘l’

310 ‘o’

311 ‘!’

312

3135

300 – divisible by 4input takes up 4 byte locations

Memory Allocation

.section “.data”input: .word 0x12345678limit: .half 0x9aprompt: .asciz “Hello!”

Address (decimal)

Data(hex)

299

300 78

301 56

302 34

303 12

304 9a

305 00

306 ‘H’

307 ‘e’

308 ‘l’

309 ‘l’

310 ‘o’

311 ‘!’

312

3136

304 – divisible by 2limit takes up 2 byte locations

Note: location 305 will have leading zeroes fill in the extra byte not specified in the data section

Memory Allocation

.section “.data”input: .word 0x12345678limit: .half 0x9aprompt: .asciz “Hello!”

Address (decimal)

Data(hex)

299

300 78

301 56

302 34

303 12

304 9a

305 00

306 ‘H’

307 ‘e’

308 ‘l’

309 ‘l’

310 ‘o’

311 ‘!’

312

3137

306 – divisible by 1Each element of prompt takes up 1 byte location

8

Addressing Variables

• Load and Store operations are the only instructions that reference memory

• Both instructions take two operands– One memory, and one register

• Can access memory using different data types (byte, half word, word, double word)

9

Load Instructions

Mnemonic Operation

ldsb Load signed byte, propagate sign left in register

ldub Load unsigned byte, clear high 24 bits of register

ldsh Load signed halfword, propogate sign left in register

lduh Load unsigned halfword, clear high 16 bits of register

ld Load word

ldd Load double, reg. # even, first 4 bytes into reg. n, next 4 into reg. n + 1

10

set input, %o0

ld [%o0], %o1 %o1 = 0x12345678

ldub [%o0 + 7], %o2 %o2 = ‘e’

ldsh [%o0 + 3], %o3 error

ldsh [%o0 + 4], %o4 %o4 = 0x9a

ldsb [%o0 + 4], %o5 %o5 = 0xffffff9a

11

Store Instructions

Mnemonic Operation

stb Store low byte of register, bits 0-7, into memory

sth Store low two bytes of register, bits 0-15 into memory

st Store register

std Store double, reg. # even, first 4 bytes from reg. n, next 4 from reg. n + 1

Why don’t we have all the same options as we did for the load instructions?

Address(decimal)

Data(hex)

300

301

302

303

304

305

306

307

308

309

310

311

312

313

12

mov 300, %o0

mov 0x12345678, %o1

st %o1, [%o0]

sth %o1, [%o0 + 6]

sth %o1, [%o0 + 9]

stb %o1, [%o0 + 13]

78563412

7856

error

78

Address(decimal)

Data(hex)

300

301

302

303

304

305

306

307

308

309

310

311

312

313

13

mov 0x9abcdef0, %o2

mov 0x87654321, %o3

std %o2, [%o0 + 4]

78563412

f0debc9a21436587

Rules to Remember

1. Lower byte Lower address

2. Reference is to the lowest address

3. Memory references must be byte aligned

14

Data Section

.section “.data”first: .word 0x03, 0x0ef321second: .byte 0x5c.align 2third: .half 0x987, 0x7estring: .asciz “done”

Pointer Address data

first 400 03

401 00

402 00

403 00

404 21

405 f3

406 0e

407 00

second 408 5c

409 undef

third 410 87

411 09

412 7e

413 00

string 414 ‘d’

415 ‘o’

416 ‘n’

417 ‘e’

418 0

419

Allocating Space

.skip is a psedo-op that will provide space without any initialization

my_array: .skip 4*100

Provides space for a 100-word unintialized array

16