ECE 447: Lecture 16

ECE 447: Lecture 16

Common Errors

& Good Programming Style

Typical Assembly LanguageProgram Bugs

ECE 447: Typical Assembly Language Program Bugs

1. Improper transfer to subroutines

Correct: JSR, BSRIncorrect: JMP, BRA

2. Forgetting to initialize stack pointer

section .text

lds #stack_end-1

done by default by the startup code when you call your assembly language routine from the main() function written in C

section .bss rmb 128stack_end


3. Not allocating enough memory for the stack

data

stack

data

stack

programexecution


4. Unbalanced stack operations

immediately after JSR just before RTS

RTNRTNSP

SPvariables


5. Using subroutines that change registers

LDX #ADDRESS

JSR changer

LDAA 0,X

Example:

6. Transposed registers

TBA vs. TAB

PSHAPSHBPULX

Examples:

instead ofPSHBPSHAPULX


7. Not initializing pointer register

8. Not initializing registers and data areas

section .bssvar1 rmb 2

LDAA 0,X

section .text

LDD var1

Example:

Example:


9. Inadvertent modification of the condition code register

CPX #end_addressLDD resultBNE start

Examples: CLCstart LDAA 0,X ADCA 0,Y STAA 0,X INX CPX #end BNE start

modifies Z flag

modifies C flag


10. Using the wrong conditional branch instruction

BHI, BHSBLO, BLS

for unsigned numbers

Correct:

BGT, BGEBLT, BLE

for signed numbers

ECE 447: Branch Instructions (1)

REL N Z V C– – – –

after comparison register vs. memory

unsigned numbers signed numbers

BHI higher >BLO lower <

BHS higher or same BLS lower or same

BGT greater than >BLT less than <

BGE greater than or equal BLE less than or equal

BEQ equal =BNE not equal


11. Using the wrong addressing mode

Examples:

LDD INIT

INIT EQU 1 var1 fdb 5

LDD #var1

instead of

LDD #INIT

instead of

LDD var1


12. Using a 16-bit counter in memory

counter fdb 0,0

inc counter

Example:

increments only the more significantbyte of a 16-bit counter!

Good Programming Style

ECE 447: Do Not Use Magic Constants!

// set EGA

PIOC |= EGA;

// clear STAI

PIOC &= ~STAI;

PIOC |= 0x40;

PIOC &= 0xBF;

Bad style Good style

#include <hc11e9.h> #include <hc11e9.h>




while (!(TFLG2 & TOF)) ;

while ((TFLG2 & 0x80)==0) ; while ((TFLG2 & TOF) == 0) ;

// do nothing while TOF equal to 0

or or

while (!(TFLG2 & 0x80)) ;



PIOC_STAI EQU $40PIOC_EGA EQU $20

// set EGA

LDAA PIOCORAA #EGASTAA PIOC

// clear STAI

LDAA PIOCANDA #~STAISTAA PIOC

PIOC EQU $1002

LDAA $1022ORAA #$02STAA $1022

LDAA $1022ANDA #$BFSTAA $1022

ECE 447: Initializing global variables

Bad practice Good practice

int counter = 0;char lookup[3];

int counter;

void main(void){ counter = 0;

}

const char lookup[] = {0xE6, 0xE4, 0xEA};const char string1 = “Menu:”;

main(){ lookup[0] = 0xE6; lookup[1] = 0xE4; lookup[2] = 0xEA;}

char string1[5]= {‘M’,’e’,’n’,’u’,’:’};

ECE 447: Clearing Timer Flags

TMSK 1 &= ~OC2F;Clears all flags in the samebyte except OC2F

or

TMSK1 |= OC2FClears all flags in the samebyte, including OC2F

Bad Good


TMSK1 = OC2FClears only OC2F

ECE 447: Common errors regarding interrupts

C

ASM

• No __attribute__((interrupt)) in the declaration/prototype of an interrupt service routine

• Using RTS instead of RTI

• Using local auto variables to store information between interrupts• Calling an interrupt service routine in your program (allowed only for testing after commenting __attribute__((interrupt)) modifier)

ECE 447: Common errors regarding interrupts

C & ASM

• No CLI• No setting of the local interrupt enable flag• Waiting for a flag to be set INSIDE of an interrupt

ECE 447: Common errors regarding polling

C• Using if instead of while to wait for a flag to be set• No clearing the flag after it was set• No using of special procedure to clear the flag• Not disabling interrupts• Checking the wrong flag

ECE 447: Common errors regarding polling

ASMUsing too complicated procedure to check the flag

LOOP LDAA PIOC ANDA #STAF

CMPA #0 BEQ LOOP

LOOP TST PIOC BPL LOOP


ECE 447: Use but not abuse comments

LDAA #INIT ; load accumulator A with ; the constant INITSTAA COUNTER ; store accumulator A to the variable COUNTER

Useless and repetitive

Effective

; initialize COUNTER to INITLDAA #INITSTAA COUNTER

VHDL: - writing synthesizable RTL level code in VHDL - writing test benches

FPGAs: - architecture of FPGA devices - tools for the computer-aided design with FPGAs - current FPGA families & future trends

Topics

ECE 448, FPGA and ASIC Design with VHDL

Applications: - basics of computer arithmetic - applications from communications, cryptography, digital signal processing, bioengineering, etc.

- FPGA boards- microprocessor board–FPGA board interfaces: PCI, PCI-X, PCIe, JTAG, USB- reconfigurable computers

High-level ASIC Design: - standard cell implementation approach - logic synthesis tools - differences between FPGA & standard-cell ASIC design flow

New trends:- using high-level programming languages to design hardware- microprocessors embedded in FPGAs

Platforms:

Tasks of the course

Advancedcourse on digital

system designwith VHDL

Comprehensive introduction to

FPGA & front-end ASIC

technology

Testing equipment

- writing VHDL code for synthesis- design using finite state machines and algorithmic state machines- test benches

- hardware:Xilinx FPGAs,TSMC libraryof standard ASICcells

- software:VHDL simulatorsSynthesis toolsXilinx ISE

- oscilloscopes- logic analyzer

Celoxica RC10 Educational Board

Digital system design technologiescoverage in the CpE & EE programs at GMU

Microprocessors ASICsFPGAs

ECE 445

ECE 447

ECE 586

ECE 681

ECE 448

ECE 511

ECE 611

ECE 431Computer Organization

Single ChipMicrocomputers

FPGA and ASIC Design with VHDL

Digital Circuit Design

Microprocessors

Advanced Microprocessors

Digital Integrated Circuits

VLSI Design Automation

ECE 545 Introduction to VHDL

ECE 645 Computer Arithmetic

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ECE 447: Lecture 16

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