ECE 353 Introduction to Microprocessor Systems Michael G. Morrow, P.E. Week 5

Flags Register Bit manipulation Logical Instructions Shift/Rotate Instructions Branching Conditional Unconditional Looping Structured Programming Stack Allocation and Operation Objectives

FLAGS Register aka 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) OFDFIFTFSFZF--AF--PF--CF

PSW Status Flags The 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 Flags The 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 Instructions Logical instructions operate bit-wise. NOT does not affect flags. MnemonicOperandsFunction OSZAPC NOTdst Logical complement ANDdst,src Logical AND 0  ?  0 ORdst,src Logical OR 0  ?  0 XORdst,src Logical exclusive OR 0  ?  0 TESTdst,src Non-destructive AND 0  ?  0

Bit Manipulation Clearing 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 issuesissues

Shift Instructions Arithmetic versus logical shifts Shift count source 1 CL Immediate byte (new to instruction set) MnemonicOperandsFunction OSZAPC SHLdst, cnt Shift logical left **  ?  SALdst, cnt Shift arithmetic left **  ?  SHRdst, cnt Shift logical right **  ?  SARdst, cnt Shift arithmetic right **  ? 

Rotate Instructions Rotate count sources Using rotate instruction to swap nibbles Execution time dependent on count MnemonicOperandsFunction OSZAPC ROLdst, cnt Rotate left ** ----  RORdst, cnt Rotate right ** ----  RCLdst, cnt Rotate through carry left ** ----  RCRdst, cnt Rotate through carry right ** ---- 

Unconditional Jumps Redirect 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 Jumps Allow program flow to change in response to conditions. Action is based on current state of flags. Syntax: j short-label is mnemonic for condition to test.condition All conditional jumps are short.  Can use double jumps if target out of range. Prior instruction must be used to set flags Examples

Looping Can 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 MnemonicOperandsFunction OSZAPC INCdst Increment  - DECdst Decrement  - CMPdst, src Compare (dst – src, nondestructive)  -

Looping Can use the LOOP instructions CX is loop control variable GPP: Don’t modify CX in loop MnemonicOperandsFunction OSZAPC LOOPshrt_lbl DEC CX JNZ LOOPE LOOPZ shrt_lbl Loop while CX <> 0 and ZF = 1 (last operation was zero) LOOPNE LOOPNZ 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 instructions CMPS order of evaluation is reversed from CMP! MnemonicOperandsFunction OSZAPC SCASdst_s Scan string (AL – ES:DI)  CMPSdst_s Compare string (DS:SI – ES:DI) 

Records Provides 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 Implementation Stack 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:SP Allocating stack space Stack operations All stack operations are 16-bit transfers PUSH / PUSHA / PUSHF POP / POPA / POPF CALL / RETURN ENTER / LEAVE Example

Wrapping Up Homework #3 due Friday, October 12 th Exam 1 will be held on Thursday, October 18, 2001 from 7:15 to 8:45 PM in 132 Noland

Exercise Write 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 for DS: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.

MnemonicJump if conditionFlags for condition JA/JNBEAbove / not below or equal(CF OR ZF) = 0 JAE/JNBAbove or equal / not belowCF = 0 JB/JNAEBelow / not above or equalCF = 1 JBE/JNABelow or equal / not above(CF OR ZF) = 1 JCCarryCF = 1 JCXZJump if CX equal 0CX = 0 (uses register) JE/JZEqual / zeroZF = 1 JB/JLNEBelow / not less nor equal((SF XOR OF) OR ZF) = 0 JGE/JNLGreater or equal / not less(SF XOR OF) = 0 JL/JNGELess / not greater nor equal(SF XOR OF) = 1 JNCNo carryCF = 0 JNE/JNZNot equal / not zeroZF = 0 JNONot overflowOF = 0 JNP/JPONot parity / parity oddPF = 0 JNSNot sign (positive)SF = 0 JOOverflowOF = 1 JP/JPEParity / parity evenPF = 1 JSSign (negative)SF = 1

Read-Modify-Write Issues Remove bubbles for memory- mapped I/O.