HACK Virtual Machine: Stack, Control, Implementation

Question 1

where does a virtual machine reside in computer strucutre

Answer 1

high level language & OS -> VM code -> machine language

Question 2

what are the 2 compilation paradigms

Answer 2

1. one-tier
2. two-tier

Question 3

what is one-tier complimation

Answer 3

source code -> compiler -> machine level instructions
- C/C++

Question 4

pros of one-tier complimation

Answer 4

1. generates efficient code
2. can take full advantage of hardware

Question 5

cons of one-tier compilation

Answer 5

1. requires one compiler per target platform
2. porting & maintaining software is time consuming

Question 6

what is two-tier compilation

Answer 6

source code -> compiler -> intermediate code -> VM translator -> executable code
- python, java, C#

Question 7

pros of two-tier compilation

Answer 7

1. combile once, run everywher
2. maximum portability

Question 8

cons of two-tier compilation

Answer 8

1. requires VM translator per target platform
2. generates less efficient machine code

Question 9

4 uses of a stack-based VM

Answer 9

1. data movement
2. arithmetic operations
3. logical operations
4. function calls

Question 10

why is a stack-based VM a simpler paradigm

Answer 10

• no registers
• no direct RAM access

Question 11

stack-based VM operations

Answer 11

1. push
2. pop

Question 12

9 stack-based VM commands

Answer 12

1. add - x + y
2. sub x - y
3. neg - -y
4. gt - x > y
5. lt - x < y
6. and - x AND y
7. or - x OR y
8. not - NOT y
9. eq - x = y

Question 13

what are the 8 default segements of the HACK VM

Answer 13

1. constant
2. static
3. local
4. argument
5. this
6. that
7. pointer
8. temp

Question 14

what is the constant segment

Answer 14

push constants to stack
- no pop allowed

Question 15

what is the static segment

Answer 15

static variables seen by a function - not used

Question 16

what is the local segment

Answer 16

a functions local variables - used for function calls

Question 17

what is the argument segement

Answer 17

a functions arguments - used for function calls

Question 18

what is the this segment

Answer 18

variable segment, starting at pointer[0] - used for objects/arrays

Question 19

what is the that segment

Answer 19

variable segment, starting at pointer[0] - used for objects/arrays

Question 20

what is the pointer segment

Answer 20

start of segments this & that

Question 21

what is the temp segment

Answer 21

segment containing eight temporary variables

Question 22

what are the 3 branching commands

Answer 22

1. label
2. goto
3. if-goto

Question 23

what does the label command do

Answer 23

defines the destenation for goto and if-goto commands

Question 24

what does the goto command do

Answer 24

jump to execute the command just after <label></label>

Question 25

what does the if-goto command do

Answer 25

pop condition from stack - if condition is true, jump to command just after - if condition is false, move to next command

Question 26

what are the 3 function commands

Answer 26

1. function 2. call 3. return

Question 27

what does the function command do

Answer 27

function - function declaration starts here

Question 28

what does the call function do

Answer 28

call - call function , informimg it that arguments have been pushed into the stack - caller MUST push values onto the stack first

Question 29

what does the return command do

Answer 29

return - return replaces arguments in the caller stack. control is transferred back to the caller. execution resumes from after the call command. - callee must push return value onto the stack before returning

Question 30

syntax for argument, constant & local memory

Answer 30

- push constant - pop argument - push local

Question 31

how is static memory assembled

Answer 31

assembler will map variable @var to the static segment & numbered within their file

Question 32

how is temp memory segment implemented

Answer 32

numbered within the whole program

Question 33

how is pointer memory implemented

Answer 33

- pop pointer i - push pointer i

Question 34

how is the call command implemented

Answer 34

1. push arguments onto the stack 2. save the return address 3. save the caller's segement pointers 4. repostion ARG 5. repsoition LCL 6. go to execute the callee's code

Question 35

how is the function command implemented

Answer 35

1. push 0s to initialise local segment 2. execute code 3. push result onto stack 4. call return

Question 36

how is the return command implemented

Answer 36

1. copy return value to caller's stack 2. recycle memory used by callee 3. reinstate caller's segment pointers 4. jump to return address