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Nick Robertson

Assembeler Class: Take home test

1.What are the four steps of CPU execution are:

1) Fetch op code or immeiate data

2) increment IP

3) decode op and or execute

4) go to step 1

2.List all the 8086 register state their prupose. Group them

by general purpose, pointing, and segment. They are:

General Registers:

axAccumilator (general purpose storage)

bxPointer to memory

cxCounting (input/output addresses)

dxData (store info about peripherals

The main purpose of the above started registers is to contain

the initaial data for, and the reslts of, arithmetic or logical


Pointing Registers:

diDdiestintaion index

siSource index

spstack pointer

bpBase pointer

The main purpose of the above stated registers is to point

to other areas in memory.


sc Code segment

dsData segment

ss Extra segment

The main purpose of the above stated registers is to hold

segments of information.

3) List all the 8086 flags and state their purpose. Group them by

by state or control, they are:

State Flags:

State flags are so named becouse their values are influenced

by the precious intructions and reflect some pecularities as a result.

AFAuxiliary CarryFlag

CFCarry Flag

OFOverflow Flag

PDParity Flag

SFSign Flag

ZFZero Flag

Control Flag:

Control Flags are so named becouse their pupose is to control

the execution of certain intructions.

IFInterrupt Flag

TFTrap Flag

DFDirection Flag

4) Define Ram and Rom

ROM – Read Only Memory


- non-volitale ( is kept at shut-down)

RAM – Random Access Memory


-Volitable ( is not kept at shut-down)

5)State and define the three busses used by the CPU.

also state if the bus is unidirectional or bidirectional.

They are:



Data -Bidirectional

Sends info ( Carrys info as well)


Mouse, Keyboard, etc……

6)A program that will take a single digit numeric

input from the computer, convert it to BCD, ass the number,

and output the result to the monitor.


.stack 100h



mov ah, 01h

Int 21h

mov ax, 001h

cmp al, 09h

Jnz convert; jnz = jump not zero

mov ax, 4C00h

int 21h

end start


add ax, 01h; add 1 if over 9


7)State the difference between a macro and a procedure.

Macro – Copies code a compilating / inserts

Procedure – Jumps to instruction in memory

8)Give exaples of the following addresing modes:

Register – mov ax, cx

Indirect – mov ax, 6000h

mov ds, ax

Direct – mov ax, [0123h]

Indexed – mov ax, [bx + di]

9)Define the following assembler directives:

Extern – External to current code

Public – Can be called (internal)

Global – Combination of both

10)For every ‘push’ there is a ‘pop’

11)Write a program that will encrypt a string by XORing one character

with the next character in the string.

.Model tiny


grab db “this”



mov ax, @ data

mov ds, ax

lea bx, grab

lea si, grab


inc si

xor [bx][si];[T][H]

inc bx

comp si, 03h

jne stinc

mov ax 4ch

int 21

end start

12)Write a program that will decrypt the program you just wrote.

.Model tiny


grab db “this”



mov ax, @ data

mov ds, ax

lea bx, grab

lea si, grab

lea dx, toinc

add bx, 04h

add si, 04h


dec si

xor [bx][si]

dec bx

cmp si, dx

jne stinc

int 21

end start

13)State the address 5F32A of the next op-code as a segmented address, a logical address. Give the offset address and the segment address.

5F32and shift to right

500000= F32A

14)List all the base-pointers registers and give the segmants that they can use. Label the default segmen register with each. Don’t forget about IP

– Ip must pair with cs, sp with ss becouse these regiesters are involved in automatic CPU operations.Then you can pair the general-purpose data pointing registers BX, Di, and SI with the data segment register DS

– The IP offset register is always asses to the CS segment register when addressing code memory. There is override available to the programmer.

– The BP or SP offset register is automatically added to the SS segment register when stack opertions are in progress. The programmer may not override the automatic selection of SS with SP. The programmer may override the automatic selection of the DS register and use anyone of the other three segment registers CS, ES, or SS.

– Any offset quantity using registers BX, DI, and Si is automaticlly added to the DS base register. The programmer may override the automatic selection of the DS register and use any one of the other three segment registeres CS, ES, or SS.

– The offset register DI is automaticlly used with the ES base register when stringoperations are taking place. The programmer may not override the selection of Di with ES when string operations are done.

15)Give an example of a struct that will store the data for a person age,

height, and weight.


age dw 0h

height+dw 0h

weight+dw 0h

health ends

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