instruction set

MACHINE CODE



Layout of bits in Format I is shown below. There are individual

fields, they are:

• Opcode: operation code. Highest 12 values for format I and rest for jump and format II

• S-reg, D-reg: 4 bits. Specifies the CPU registers.

• As: 2 bits. Gives mode of addressing for the source

• Ad: 1 bit gives mode of addressing for destination

Format 2:

Single operand

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

Op-code b/w Ad D/S-reg

Jump instruction

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

Op-code Condition 10-bit, 2’s complement PC offset

• There are 7 addressing modes for the source.

• There are 4 addressing modes for destination of which, 2 are emulated

•_{For destination:}

_{Register mode; Indexed mode; Symbolic mode;}

EXAMPLE:

Instruction: 1.) mov.w R5,R6

Op-code

mov S-regr5 RegisterAd 16-bitsb/w RegisterAs D-regr4

0 1 0 0

0 1 0 1

0

0

0 0

0 1 10

Encoded value: 0x4506 2.) add.w R5,R6

3.) mov.w #5, R6;

For double operand instruction ( labels)

mov.w r5,TONI

Op-code

mov S-regr5

Ad Symboli

c

b/w

16-bits RegisterAs D-regPC

0 1 0 0

0 1 0 1

1

0

0 0

0 0 0 0

2’s complement PC-relative destination index



Copy the contents of a PC-relative memory location to

another PC-relative memory location



Assembly:

mov.b EDEN,TONI

Instruction code:

0x40d0



Three word instruction



The CPU copies the 8-bit contents of EDEN (pointed to by

source index + PC

) to TONI (pointed to by

destination index + PC

)

Op-code

mov S-regPC SymbolicAd 8-bitsb/w SymbolicAs D-regPC

0 1 0 0

0 0 0 0

1

1

0 1

0 0 0 0

2’s complement PC-relative source index



Logically shift the contents of register

r5

to the right through the

status register carry



Assembly:

rrc.w r5



Instruction code:

0x1005

Op-code

rrc

16-bits

b/w

Register

Ad

D-reg

r5

0 0 0 1 0 0 0 0 0

0

0 0

0 1 0 1

Assembly:

rra.b &P2OUT

Instruction code:

0x1152

Op-code

rra

8-bits

b/w

Indexed

Ad

D-reg

r2

0 0 0 1 0 0 0 1 0

1

0 1

0 0 1 0

Absolute memory address (P2OUT)

JUMP

INSTRUCTIONS

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

Op-code Condition 10-bit, 2’s complement PC offset



Jump instructions are used to direct program flow to

another part of the program.



The condition on which a jump occurs depends on the

Condition field consisting of 3 bits:

 000: jump if not equal  001: jump if equal

 010: jump if carry flag equal to zero  011: jump if carry flag equal to one  100: jump if negative (N = 1)

 101: jump if greater than or equal (N = V)  110: jump if lower (N  V)



Continue execution at the label

main

if the carry bit is set

Assembly:

jc main



Instruction code:

0x2fe4



One word instruction



The CPU will add to the

PC

(R0) the value

-28 x 2

if the

carry is set

Op-code

JC

Condition

Carry Set

10-Bit, 2’s

complement PC

offset

-28

ILLEGAL OPERATIONS

Fetching data:

˚

data fetched from non-existent region is random.

˚

Data for word must be aligned to even address.

˚

Peripheral registers for word access, meant for read only as

words.

Illegal instructions:

•

Instructions are 16-bit word. Not all 2

are used.

•

Ranges 0x0000-0x0FFF and 0x1000-0x1FFF are not used.

•

0x0FFFF, illegal opcode trap. Program wanders into un-

programmed flash memory.

Fetching data

:

•

Legit to read from flash or Ram but not peripheral register.

•

It is not preferred to read from non-existent memory, causes

reset.

REVIEW

Instruction:

mnemonic src,dst ;

• Header file : #include<msp430XX.h>

• Relocating segments: RSEG / ASEG

• File : *.s43

• Use infinite loop as there is no operating system to take over in a relatively small embedded system.

EXAMPLES

• Basic program is the hello world program.

#include <stdio.h> void main (void)

{

printf("hello , world\n"); }

• Back then, the system would send the printf() to a printer or any display device.

• It can also be displayed on character LCD, but simpler to turn on the LEDs.

LEDS

#include <msp430x11x1.h> ; Header file for this device

ORG 0xF000 ; Start of 4KB flash memory

Reset: ; Execution starts here

mov.w #WDTPW|WDTHOLD ,& WDTCTL ; Stop watchdog timer

mov.b #00011000b,& P2DIR ; Set pins with LEDs to output

mov.b #00001000b,& P2OUT; LED2 (P2.4) on , LED1 (P2.3) off (active low!)

InfLoop: ; Loop forever ...

jmp InfLoop ; ... doing nothing

;---ORG 0xFFFE ; Address of MSP430 RESET Vector

#include <msp430XX.h> // Specific device

void main (void)

{

WDTCTL = WDTPW | WDTHOLD; // Stop watchdog timer

P2DIR = 0x18; // Set pins with LEDs to output , 0b00011000

P2OUT = 0x08; // LED2 (P2.4) on , LED1 (P2.3) off (active low!)

for (;;)

{ // Loop forever ...

} // ... doing nothing

COPYING STRINGS

ALP to copy string

MyStrCpy: jmp CopyTest

CopyLoop: mov.b @R14+,0(R12) inc.w R12

CopyTest: tst.b 0(R14) jnz CopyLoop

clr.b 0(R12) terminate string

ret

Copying a string in C

void MyStrCpy(char * dest , const char * src) {

while (*src != '\0') // test for end of string {

*dest++ = *src ++; // copy nonnull character

}

*dest = '\0'; // terminate destination string

Copying a block of data between fixed addresses.

mov.w #BeginSource ,R14 ; Load address of source

jmp CopyTest CopyLoop:

mov.b @R14+,BeginDest -BeginSource+0 xFFFF(R14) CopyTest:

cmp.w #EndSource ,R14 ; Set flags for (R14 - end of source)

jlo CopyLoop

jmp $ ; Infinite , empty loop

RSEG DATA16_C ; Segment for constant data in ROM

BeginSource: ; Source data

DB "hello , world\n“ ; "" causes a '\0' to be appended

Reading a value from push button

ALP

bit.b #BIT1 ,&P2IN ; Test button on bit 1 of P2IN; C = ˜Z

rrc.b ShiftReg ; Insert C into msb of shift register In C

ShiftReg >>= 1; // Shift register right by 1 position

if (P2IN_bit.P2IN_1 == 1)

{ // Test button on bit 1 of P2IN

ShiftReg |= BIT7; // Set msb of byte

} else {

ShiftReg &=˜BIT7; // Clear msb of byte

REFLECTION ON CPU

INSTRUCTION SET

•

The MSP430 was designed from the start for low power

consumption, computing in very few clock cycles

•

Most programs today are written in C so the processor is

designed for efficient compilation

•

an advantage of a 16-bit processor is that registers can be

used for either data or addresses.

•

Moreover, most ADC now produce more than 8 bits of

output ,so it is more efficient to handle words rather than

bytes.

IS MSP430 RISC?

Registers of CPU

• a straightforward set of 16 registers for addresses and data.

• The constant generator is a particularly creative feature.

Is MSP 430 a RISC?

Four main features of a RISC processor:

Small set of general-purpose instructions Large bank of general-purpose registers Load–store architecture

Single-cycle execution

Ex: clearing control bit in the timer

bic.w #MC0|MC1 ,& TACTL ; stop timer [3 words , 5 cycles]

Also

load.w #TACTL ,R4 ; load address of TACTL [2w, 2c]

load.w @R4 ,R5 ; load value of TACTL [1w, 2c]

load.w #MC0|MC1 ,R6 ; load immediate operand [2w, 2c]

bic.w R6 ,R5 ; perform operation [1w, 1c]

Addressing Modes

• A Format I instruction with two operands must fit into a 16-bit word

• 4 bits for operation, 4 bits for dst, src regester and 1 bit for B/W, leaving only 3 bits left.

• 2 for src, 1 for dst. This makes use of constant generator to give different modes for source easily.

• The lack of indirect or autoincrement modes for the destination can be frustrating when writing assembly language

• A missing mode for addressing is predecrement, useful to push data onto stack

Conclusion

• MSP430 has orthogonality.

• Access to memory is simple and uniform.

• The constant generator makes programs faster and more compact.

• It is extremely easy to forget the # when calling subroutines in the most straightforward way and the results are disastrous.

EXTRAS

For an LED,

Toggling bits P1OUT |= BIT4;

To turn it off, we give: P1OUT &= ~BIT4;

To toggle its state (off if on, on if off): P1OUT ^= BIT4;

Coding Interrupts

Every ISR has this structure:

#pragma vector = <VECTOR_NAME> __interrupt void <ISR_NAME> (void) {

BUTTON

PRESS

#include <msp430XX.h> #define red_LED BIT0 #define grn_LED BIT6 #define BTN BIT3 void delay(void);

void main(void) { unsigned int flash;

WDTCTL = WDTPW + WDTHOLD; P1OUT = 0;

P1DIR |= red_LED + grn_LED; // LED pins to outputs, BTN is // still an input by default.

for (;;) {

for (flash=0; flash<7; flash++) {

P1OUT |= red_LED; // red LED on delay(); // call delay function P1OUT &= ~red_LED; // red LED off delay(); // delay again

}

while ((P1IN & BTN) == BTN); // wait for button press for (flash=0; flash<7; flash++) {

P1OUT |= grn_LED; // green LED on delay();

P1OUT &= ~grn_LED; // green LED off delay();

}

while ((P1IN & BTN) == BTN); // wait for button press }