ECE 353Introduction to Microprocessor Systems

Michael G. Morrow, P.E.

Week 5

Flags RegisterBit manipulation Logical Instructions Shift/Rotate Instructions

Branching Conditional Unconditional

LoopingStructured ProgrammingStack Allocation and Operation

Objectives

FLAGS Registeraka the Processor Status Word (PSW)A ‘flag’ is generally a marker used to indicate or record some condition.PSW contains 6 status flags … AF, CF, OF, PF, SF, ZF

… and 3 control flags DF, IF, TF

80C188EB Processor Status Word (PSW)

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

-- -- -- --OF

DF

IF TF SF ZF --AF

-- PF -- CF

1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0

PSW Status FlagsThe status flags reflect the result of the previous logical or arithmetic operation. AF – indicates a carry or borrow between the

high and low nibbles of the accumulator, used for BCD.

CF – indicates a carry from, or a borrow to, the MSb of an arithmetic result Can also modify directly with CLC / CMC / STC to use as

a Boolean status bit. OF – an arithmetic overflow has occurred PF – set if the operation resulted in even parity SF – set if the result is negative (i.e. MSb = 1) ZF – set if the result is zero

Control FlagsThe control flags control certain aspects of the processor’s execution DF – direction flag

Determines the direction of pointer modifications in string operations. If DF=0, then increment pointers, otherwise decrement.

IF – interrupt enable flag If set, the processor can recognize

maskable interrupts. TF – trap flag

If set, the processor will execute in single-step mode.

Logical InstructionsLogical instructions operate bit-wise.NOT does not affect flags.

Mnemonic

Operands

Function O S Z A P C

NOT dst Logical complement

- - - - - -

AND dst,src

Logical AND 0 ? 0

OR dst,src

Logical OR 0 ? 0

XOR dst,src

Logical exclusive OR

0 ? 0

TEST dst,src

Non-destructive AND

0 ? 0

Bit ManipulationClearing bit(s) - AND Set desired bits to 0 in mask All other bits in mask to 1

Setting bit(s) - OR Set desired bits to 1 in mask All other bits in mask to 0

Toggling bit(s) - XOR Set desired bits to 1 in mask All other bits in mask to 0

Read-modify-write issues

Shift InstructionsArithmetic versus logical shiftsShift count source 1 CL Immediate byte (new to 80186 instruction set)

Mnemonic

Operands

Function O S Z A P C

SHL dst, cnt

Shift logical left * ?

SAL dst, cnt

Shift arithmetic left * ?

SHR dst, cnt

Shift logical right * ?

SAR dst, cnt

Shift arithmetic right

* ?

Rotate InstructionsRotate count sourcesUsing rotate instruction to swap nibblesExecution time dependent on count

Mnemonic

Operands

Function O S Z A P C

ROL dst, cnt

Rotate left * - - - -

ROR dst, cnt

Rotate right * - - - -

RCL dst, cnt

Rotate through carry left

* - - - -

RCR dst, cnt

Rotate through carry right

* - - - -

Unconditional JumpsRedirect program flow by loading a new IP (and possibly a new CS) value.Syntax: jmp target Label attributes Target by direct addressing Target by indirect addressing

Jump tables Be sure index is bounds-checked! FAR versus NEAR jump tables

Conditional JumpsAllow program flow to change in response to conditions.Action is based on current state of flags.Syntax: j<X> short-label <X> is mnemonic for condition to test. All conditional jumps are short.

Can use double jumps if target out of range.

Prior instruction must be used to set flags Examples

LoopingCan use backwards conditional jumps to create a loop that can be terminated - By a condition After a predetermined number of iterations Or a combination of both

Mnemonic

Operands

Function O S Z A P C

INC dst Increment -

DEC dst Decrement -

CMP dst, src

Compare (dst – src, nondestructive)

-

LoopingCan use the LOOP instructionsCX is loop control variable GPP: Don’t modify CX in loop

Mnemonic Operands

Function O S Z A P C

LOOP shrt_lbl

DEC CXJNZ

- - - - - -

LOOPELOOPZ

shrt_lbl

Loop while CX <> 0 and ZF = 1 (last operation was zero)

- - - - - -

LOOPNELOOPNZ

shrt_lbl

Loop while CX <> 0 and ZF = 0 (last operation was not zero)

- - - - - -

Implementing Structured Programming Constructs

Structured programming basics One entry point, one exit point per

procedure (subroutine) Based on three basic control structures

Repetition Selection

If, If/Else, Switch/Case

Repetition While Do-While

Flowchart Basics

Repeated String Instructions

String instructions become very useful when used with the REP prefix REP, REPE/REPZ, REPNE/REPNZ

Pointer modification based on DF (CLD, STD).CX is always loop variable for repeated string instructionsCMPS order of evaluation is reversed from CMP!

Mnemonic

Operands

Function O S Z A P C

SCAS dst_s Scan string(AL – ES:DI)

CMPS dst_s Compare string(DS:SI – ES:DI)

RecordsProvides a syntax for creating and accessing bit-encoded data structures.Syntax name RECORD field_name:exp[=init_val][,…]

Operations MASK

Creates a mask for the bit-field identified Shift

Using the bit-field name is interpreted as a right shift count to move that field to the LSB

WIDTH Returns number of bits in a record or field

Stack ImplementationStack is a LIFO data structure.Who uses the stack? Subroutine return addresses Passed parameters Temporary memory allocation

Two basic operations PUSH POP

Hardware stacks vs. memory stacks Stack pointer

A dedicated register to use exclusively to access the stack

80C188EB Stack Operation

Stack is defined by SS:SPAllocating stack spaceStack operations All stack operations are 16-bit

transfers PUSH / PUSHA / PUSHF POP / POPA / POPF CALL / RETURN ENTER / LEAVE

Example

Wrapping UpHomework #3 due Friday, October 12th

Exam 1 will be held on Thursday, October 18, 2001 from 7:15 to 8:45 PM in 132 Noland

ExerciseWrite a code fragment that implements the C function strchr, which finds a given character in an ASCIIZ string.strchr scans the string in the forward direction, looking for the specified character and finds the first occurrence of the character in the string. The null-terminator is considered to be part of the string.

Assume the following conditions: AL - character to search forDS:DI - address of null-terminated string to search

(string must not start at offset of zero!)If found, set AX = equal offset of first occurrence, otherwise set AX = 0.

Mnemonic Jump if condition Flags for conditionJA/JNBE Above / not below or equal (CF OR ZF) = 0

JAE/JNB Above or equal / not below CF = 0

JB/JNAE Below / not above or equal CF = 1

JBE/JNA Below or equal / not above (CF OR ZF) = 1

JC Carry CF = 1

JCXZ Jump if CX equal 0 CX = 0 (uses register)

JE/JZ Equal / zero ZF = 1

JB/JLNE Below / not less nor equal ((SF XOR OF) OR ZF) = 0

JGE/JNL Greater or equal / not less (SF XOR OF) = 0

JL/JNGE Less / not greater nor equal (SF XOR OF) = 1

JNC No carry CF = 0

JNE/JNZ Not equal / not zero ZF = 0

JNO Not overflow OF = 0

JNP/JPO Not parity / parity odd PF = 0

JNS Not sign (positive) SF = 0

JO Overflow OF = 1

JP/JPE Parity / parity even PF = 1

JS Sign (negative) SF = 1

Read-Modify-Write Issues

Remove bubbles for memory-

mapped I/O.