ECE 353Introduction to Microprocessor Systems

Michael J. Schulte

Week 4

Data Transfers and AddressesAddressing ModesMemory AllocationStructuresStringsTable Look-upAddress Object TransfersMemory Alignment Considerations

Topics

Data TransfersInternal vs. External Registers Each register in the processor (internal) as well

as memory and I/O registers (external) has an address – they’re just not all in the same space.

Logical Addresses Registers I/O Memory

Addressing Modes Addressing modes determine how and when

operands can be accessed in registers, I/O space, and memory space.

Instructions of the WeekIN acc, portOUT port, accINS dst_s, portOUTS port, src_sMOV dst, srcXCHG dst, srcMOVS dst_s, src_sLODS src_sSTOS dst_sXLAT translate_tble

Addressing ModesImmediate Addressing The operand is encoded within the instruction

itself Immediate operands are constants, so they must

be the source operand, not the destination Values are sign-extended as required ASM86 numeric notation suffixes – B, Q or O, H Defined constants (EQU) Character constants Assembler expressions

Allow assembler to do some low level calculations Conditional assembly

MOV instruction

Addressing ModesRegister Addressing The operand is one of the 80C188EB’s

internal registers. Register operand encoding only

requires 3-bits plus w bit, so shorter instructions.

Does not run an external bus cycle No EA calculation No R/W cycle

XCHG Instruction

Addressing ModesI/O Port Addressing IN and OUT are most common I/O

instructions. AX or AL must be used as the source

(OUT) or destination (IN). The width (byte/word) of the transfer is

determined by the register choice. Physical address always has A19:16 =

0. Segment registers are not used.

Addressing ModesFixed Port Addressing (I/O) Address of I/O port is encoded within

instruction, limited to 8-bits Can only access 0000h – 00FFh

Can be used for byte or word ports 80C188EB requires two transfers to

read/write word ports I/O port address is constant, i.e. must

be known at assembly-time IN instruction

Addressing ModesVariable Port Addressing (I/O ) DX register is used to supply a 16-bit

address Can access entire I/O space Byte or word transfers

DX is not modified by instruction, just used as a pointer (like indirect addressing)

Allows for port addresses to be computed or operated on at run-time (i.e. incrementing through a range of port addresses in a loop)

Addressing ModesI/O String Port Addressing INS

Transfers from an I/O port to ES:DI OUTS

Transfers from DS:SI to an I/O port Requires two transfers, i.e. INS

transfers from I/O port to a temporary register, then the temporary register is transferred to memory

Most often used in combination with the REP modifier for low-overhead block moves

Memory AllocationMemory operand types Byte, word, double-word Stored in little-endian format

Data item attributes Type, Offset, Segment

Data allocation directives DB, DW, DD Identifiers Initializers Arrays and strings

Setting up a data segmentVariable naming

Memory Addressing ModesEvery memory location is referred to by a logical address (segment : offset)

BIU provides the segment EU supplies the offset (EA)

EA calculation is based on the addressing mode

In general, the EA is computed as: EA = [displacement] + [BX or BP] + [SI or DI] Displacement is an 8- or 16-bit constant supplied

at assembly-time In the original 8088, EA computation used the ALU

and so added a significant delay to the instruction execution time – the 80188 added dedicated address generation hardware to speed things up.

Memory Addressing ModesDirect Addressing EA is a variable’s offset within the

applicable segment EA is contained in the displacement

field of instruction (16-bit value) The address is a constant determined at

assembly-time The segment register used is based on

the instruction (default segment), the location of the label used to specify the offset, or the use of an explicit segment override prefix

Memory Addressing ModesRegister Indirect Addressing EA is contained in an index or pointer

register BX, BP, DI , SI

Remember – BP always uses SS

Getting the EA of a label The OFFSET assembler operator.

Resolving anonymous memory references The PTR assembler operator

Other assembler attribute operators

Memory Addressing ModesIndexed Addressing EA = displacement + [SI, DI, BX or BP]. Displacement is a constant calculated at

assembly-time. Typically used for accessing data in

arrays. Displacement = array starting offset Register holds (element index × element

size) If array of bytes, element size = 1 If array of words, element size = 2 If array of double-words, element size = 4

Memory Addressing ModesBased Addressing EA = [BX, BP, DI or SI] + displacement. Displacement is a constant calculated at

assembly-time. Typically used for accessing information

in data structures. Register holds starting address of structure. Displacement = offset from start of structure

to desired structure element. Code can access any instance of the

structure just by changing register contents to point at it.

Memory Addressing ModesBased Indexed Addressing EA = [BX, BP] + [DI, SI] + displacement. Displacement is a constant calculated at

assembly-time. Typically used for accessing information

in arrays of data structures. BX = starting offset of array DI = offset to desired array element Displacement = offset within a structure to

the desired element.

Memory Addressing ModesString Addressing Memory source is DS:SI

Can override to CS, ES, SS Memory destination is ES:DI String instruction primitives

INS, OUTS, MOVS, LODS, STOS The size of the operand is determined by the

data type or by appending B or W to the mnemonic.

All automatically update DI/SI based on the direction flag (DF) and operand size.

String instructions commonly use the REP prefix to do block operations (more on that later)

Creating Data StructuresStructure template STRUC / ENDS Defines a data structure, does not

allocate memory.

Structure use Allocating space for structure. Initializing structure members. Referring to structure members using

direct addressing. Creating and indexing arrays of

structures.

AddressabilityOperand addressability Definition

Using the ASSUME directive Purpose Relation to segment register contents

Segment override prefixes Assembler generated Explicit

Allocating ROM SpaceIn an embedded system, the code segment is usually in ROM.Data variables in the code segment will be placed in ROM also – so are constant. Example

Look-up tables Purpose Usage

Indexed addressing XLAT instruction

Address Object TransfersUsed to load an address into a register for indirect addressing. LEA

Calculates the EA for the source operand and stores it in the destination register.

LDS / LES Loads from a double-word in memory

pointed to by the source operand (direct/indirect/etc.).

Low-word is transferred to destination register (often SI/DI), high-word is transferred to the segment register (DS/ES).

80C186EB Alignment Issues

The 80C186EB has a 16-bit data bus.Loading a word only takes one cycle, if the word is properly aligned. If loading an unaligned word, two bus cycles are required. 80C186EB memory map

Data alignment is often an important factor to maximize the efficiency of a wide data bus.EVEN assembler directive Forces the following data to be located on a word

boundary (even address). Assembler will pad with a NOP to correct alignment.

Wrapping UpReading for next week Chapter 7, 8.1-8.2

Homework 2 due on February 18

Exam #1 will be held Wednesday, 10/13 at 7:00pm. Coverage will be over modules 1 and 2.

ExercisesCreate a source code template with a code segment and a data segment, establish addressabilityDeclare a 100 byte array in DS and initialize to 0Create a far (32-bit) pointer to the array in DSDeclare a word variable in CSCopy the segment part of the far pointer to the word variable you just declaredLoad BL with the 51st element of the array using indexed addressingFill the 99th array element with its index using based addressingExchange the 99th and 100th elements of the array

XCHG instruction

MOV instruction

XCHG Instruction

IN Instruction

OUTS instruction

c signed characteruc unsigned characteri integerui unsigned integersi short integerli long integern an integer number where the actual size is irrelevantf floatd doubles string of characters sz string of characters, terminated by a null characterb an integer or character used as a boolean valueby single bytect an integer being used as a counter or tallyp pointer to a structure or general void pointerpx pointer to a variable of class x, e.g. pi, pf, pli

Notation Trivia

ROM Variables.186assume ds:data, cs:code

data segmentDATA_VAR db 1 ;declare DS variable data ends

code segment

CODE_VAR db 2 ;declare CS variable

start: mov ax, data ;setup data segmentmov ds, axmov al, DATA_VAR;load DS variable

mov al, CODE_VAR;load CS variable

jmp startcode endsend start

ROM Variable Listing

2Eh is the CS segment override prefixr indicates that this is a relative offset within the segments in this file – the linker will calculate the actual value after it combines any other segment pieces into one segment.

……0006 A0 0000r mov al, DATA_VAR0009 2E:A0 0000r mov al, CODE_VAR……

Data AlignmentThe 80C186 memory map is logically the same as the 80C188, but it is physically different.

LSB MSB

DS:0000

34h 12h

DS:0002

? 9Ah

DS:0004

78h

DS:0006

DS:0008

DS:000A

data segmentVar1 dw 1234h

db ?Var2 dw 789Ahdata ends

Var2

Var1

SelectedAssembler Expression Operators

Operation Syntax Result

Arithmetic

High byte isolation

HIGH operandHigh byte of operand

Low byte isolation

LOW operand Low byte of operand

Shift right operand SHR count Bitwise right shift

Shift left operand SHL count Bitwise left shift

Modulusoperand MOD operand

Remainder

Logical

Logical ANDoperand AND operand

Bitwise AND

Logical XORoperand XOR operand

Bitwise XOR

Logical NOT NOT operand Bitwise complement

Relational

Equal operand EQ operandAll 1’s if true, 0 otherwise

Not equal operand NE operand

Less than operand LT operand

XLAT Instruction

Another ExerciseGiven the below variable declarations, address the middle element of VAR1 using direct, register indirect, and indexed addressing.VAR1 dw 0, 1, 2, 3, 4VAR2 dw 0, 1, 2, 3, 4VAR3 dw 0, 1, 2, 3, 4

What sort of addressing would be appropriate if you were searching… Through a single array? Looking at a certain element in all of the

arrays?