Lecture_5 Introduction to Assembly Langu

Lecture 5 : 

Lecture 5 Introduction to Assembly Language Programming

Assembly vs. High Level Language : 

Assembly vs. High Level Language Assembly is a low-level language. It is different from other high-level languages. High level languages are abstract. A single high level instruction is translated into several machine language instructions. Over time the level of abstraction of high-level language is increasing. Assembly is harder to program than high level languages. Assembly Language program can be 20 times faster than high level program. Assembly gives direct access to hardware features. Faster development time for high level programs.

Assembly vs. High Level Language : 

Assembly vs. High Level Language Unsigned int outdata=0; For (outdata=0;outdata<10;outdata++) { PortD=outdata; }

Simplest program possible : 

Simplest program possible main: ;this is the label “main” rjmp main ;Relative Jump to main “Main” is used a label. This label replaces an address in FLASH. rjmp main is places at exactly this address. Upon execution CPU will jump to “main”. Jump will be repeated over and over, resulting in an infinite loop.

Interrupt Vector Table : 

Interrupt Vector Table After power-up, or when a reset has occurred, the MC will always start program execution from address 0x0000. The first bytes in code space are the “Interrupt Vector” Table. The interrupt vector table can be used by you to tell the MC what it has to do when a specific interrupt occurs. The first interrupt vector is the “reset vector”. Upon reset we will jump to our program. At 4Mhz rjmp takes 0.0000005s since it is 2 cycles.

Flashing LED program : 

Flashing LED program We want to make an LED flash. LED will show active low behaviour After reset, all Data bits are set to zero, so the LED should be ON when the program is executed

Flashing LED program : 

Flashing LED program All PortB pins are configured as output pins.

Flashing LED program : 

Flashing LED program Basically we want to switch LED on and off in a loop. We use cbi (clear bit in I/O) and sbi (set bit in I/O). We will see LED being on all the time. I cycle for LED off then 1 cycle (0.00000025) later it is on again. Rjmp takes 2 cycles (0.0000005). We naturally need delay.

Flashing LED program : 

Flashing LED program For 0.5s delay at 4MHz equals 2,000,000 cycles. An 8-bit register can only hold values 0 to 255 so it won’t be enough. Registers can be used in pairs so we can work on values from 0 to 65535. 4(2adiw+2brne)*0xFFFF(looping) + 3(overflow 2adiw+1brne) + 2(clr) = 262145 cycles. This is still not enough: 2,000,000/262,145 ~ 7.63.

Flashing LED program : 

Flashing LED program We will create a loop “around” the inner loop. We change clr to ldi so that we can use different start value than 0. The “outer” loop will down-count from 8 to 0. The overall loop needs: 262,145 (inner loop) + 1 (dec) + 2 (brne) = 262148 * 8 = 2097184 cycles plus the initial ldi = 2097185 minus one since last brne didn’t result in a branch. This is 97184 cycles too long. We will fine-tune with initial value in r24:r25. Subtract : 97184 / 8 =12148 cycles per inner loop. Inner loop takes 4 cycles. So 12148/4=3037 less iterations. This is initial value in r24:r25.

Slide 11: 

Also add .include “m16def.inc” or any relevant file such as m8def.inc

The Stack : 

The Stack Stack is like a notepad to remind you where you just left in case you are visiting several locations. Stack pointer tells you where that stack is. When a subroutine is called, it leaves the place in flash where it was just working and saves the return address on the stack. The stack needs a stack pointer (SP) and space in SRAM (the stack pointer must point above the first SRAM address). When a return address is stored, the SP is post-decremented. i.e. the Stack is growing towards smaller SRAM addresses. Biggest stack possible is initialized to RAMEND till first SRAM location.

The Stack : 

The Stack .include “m16def.inc” .def temp = r16 .org 0x00 ldi temp, low(RAMEND) out SPL,temp ldi temp, high(RAMEND) out SPH, temp rcall subrtn_1 .org 0x100 subrtn_1: rcall subrtn_2 ret .org 0x140 subrtn_2: ret

Push/Pop : 

Push/Pop The stack can also be used to pass arguments to subroutines using push and pop The stack can also be used to pass arguments to subroutines using push and pop. Balance Push and Pop. Missing a pop or too many pops above can cause error on Subroutine return.

Stack Initialization for Subroutines : 

Stack Initialization for Subroutines

Subroutines : 

Subroutines RAMEND is defined in the micros include file you get with AVRStudio and equal to the last available internal SRAM address. A subroutine begins with a label which is the subroutine's name. .include "m16def.inc" ldi r16, low(RAMEND) out SPL, r16 ldi r16, high(RAMEND) out SPH, r16 main: rcall out_PortA rjmp main out_portA: out PortA, r16 ret Rcall: This instruction jumps to a relative address and is 2 bytes long and needs 3 cycles for execution. The disadvantage is that the subroutine has to be located at +/- 2k words. Call can be used for absolute addressing (4bytes, 4cycles). The 8k AVRs only need rjmp and rcall.