1. ECE 353 Introduction to Microprocessor Systems
Week 5
Michael J. Schulte

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

3. 80C188EB Processor Status Word (PSW)
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) 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 -- OF DF IF TF SF ZF AF PF CF

4. 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

5. 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.

6. Logical Instructions Logical 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  ? OR
Logical OR
XOR
Logical exclusive OR
TEST
Non-destructive AND

7. Questions? What logical operations would be necessary to
Check the state of a single bit?
Check if multiple bits are 1?
Check if certain bits have desired states?
Set a bit or bits?
Clear a bit or bits?
Toggle a bit or bits?

8. Bit Manipulation Clearing bit(s) - AND Setting bit(s) - OR
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
Read-modify-write issues

9. Shift Instructions Arithmetic versus logical shifts Shift count source1 CL
Immediate byte (new to instruction set)
Mnemonic
Operands
Function O S Z A P C
SHL
dst, cnt
Shift logical left *  ?
SAL
Shift arithmetic left
SHR
Shift logical right
SAR
Shift arithmetic right

10. Rotate Instructions Rotate count sources same as shifts
Using rotate instruction to swap nibbles
Execution time dependent on count
Mnemonic
Operands
Function O S Z A P C
ROL
dst, cnt
Rotate left * - 
ROR
Rotate right
RCL
Rotate through carry left
RCR
Rotate through carry right

11. 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

12. Conditional Jumps
Allow 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

13. 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
Mnemonic
Operands
Function O S Z A P C
INC
dst
Increment  -
DEC
Decrement
CMP
dst, src
Compare (dst – src, nondestructive)

14. Looping Can use the LOOP instructions CX is loop control variable O S
Warning: modifying CX in the loop can be disastrous
Mnemonic
Operands
Function O S Z A P C
LOOP
shrt_lbl
DEC CX
JNZ -
LOOPE
LOOPZ
Loop while CX <> 0 and ZF = 1 (last operation was zero)
LOOPNE
LOOPNZ
Loop while CX <> 0 and ZF = 0 (last operation was not zero)

15. Implementing Structured Programming Constructs
Structured programming basics
One entry point, one exit point per procedure (subroutine)
Based on three basic control structures
Sequence
Selection
If, If/Else, Switch/Case
Repetition
While
Do-While
Flowchart Basics

16. 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!
Mnemonic
Operands
Function O S Z A P C
SCAS
dst_s
Scan string
(AL – ES:DI) 
CMPS
Compare string
(DS:SI – ES:DI)

17. 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

18. Stack Implementation Stack is a LIFO data structure.
What uses the stack?
Subroutine return addresses
Passed parameters
Temporary memory allocation
Two basic stack operations
PUSH
POP
Hardware stacks vs. memory stacks
Hardware stack
Memory stack pointer
Stack pointer

19. 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

20. Wrapping Up Homework #3 is due Friday, March 4, 2005.
Exam #1 will be held on Wednesday, February 13th, 2005 from 7:15 to 8:45PM in room 1227 Engineering Hall.
One 8.5x11 sheet with original, handwritten notes.
Instruction set guide will be provided.
For next time week read Ch , 8.8, 9.

21. 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 initial 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.

22. Mnemonic Jump if condition Flags for condition JA/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
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
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

23. Read-Modify-Write Issues

24. PIC16F84 Hardware Stack