Class notes from week of April 2-5

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CPU Registers

R	accumulator
PC	program counter
IR	instruction register
MAR	memory address register
MDR	memory data register
CCR	comparison flags
ALU1	ALU operand 1
ALU2	ALU operand 2

Go over the function of each register.

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Assembly Language Instructions

opcode	mneumonic	meaning

0000	LOAD       X	load contents of location X into accumulator register R
0001	STORE      X	store contents of accumulator register R in location X
0010	CLEAR      X	store 0 in location X
0011	ADD        X	add contents of location X to accumulator register R
0100	INCREMENT  X	add 1 to contents of location X
0101	SUBTRACT   X	subtract contents of location X from accumulator register R
0110	DECREMENT  X	subtract 1 from contents of location X

0111	COMPARE    X	compare contents of location X to accumulator register R
			if contents(X) > R, set GT bit to 1 else 0
			if contents(X) < R, set LT bit to 1 else 0
			if contents(X) = R, set EQ bit to 1 else 0
1000	JUMP       X	store address X in program counter register PC
1001	JUMPGT     X	if GT bit = 1, store address X in program counter register PC
1010	JUMPEQ     X	if EQ bit = 1, store address X in program counter register PC
1011	JUMPLT     X	if LT bit = 1, store address X in program counter register PC
1100	JUMPNEQ    X	if EQ bit = 0, store address X in program counter register PC

1101	IN         X	input an integer value and store it in location X
1110	OUT        X	output the contents of location X
1111	HALT		stop program execution


Pseudo-ops:

.begin
LABEL:	instruction
LABEL:	.data	value
.end

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Translate into assembly code

(a) Add 1 to the value of x

        INCREMENT X

(b) Add 50 to the value of x

        LOAD    X
        ADD     FIFTY
        STORE   X
        HALT
FIFTY:  .data   50

(c) Set x to the value y+z-2

      LOAD     Y
      ADD      Z
      SUBTRACT TWO
      STORE    X
      HALT
TWO:  .data   2

(d) If x > 50 then output the value of x, otherwise input a new value of x

      LOAD     FIFTY
      COMPARE  X
      JUMPGT   YES
      IN       X
      HALT
YES:  OUT      X
      HALT
 
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Example: a program to count from 1 to 10

Pseudocode:

set COUNT to 1
while COUNT <= 10 do         (or equivalently: if COUNT > 10 quit loop)
   output COUNT
   set COUNT to COUNT+1
(end of loop)

Assembly code:

Address          Instruction
         .begin
0                LOAD       ONE
1                STORE      COUNT
2        LOOP:   LOAD       TEN
3                COMPARE    COUNT
4                JUMPGT     DONE
5                OUT        COUNT
6                INCREMENT  COUNT
7                JUMP       LOOP
8        DONE:   HALT
9        COUNT:  .data      0
10       ONE:    .data      1
11       TEN:    .data      10
         .end

Symbol table:

Data label             Address
  COUNT                  000000001001  (9)
  ONE                    000000001010  (10)
  TEN                    000000001011  (11)
Instruction label      Address
  LOOP                   000000000010  (2)
  DONE                   000000001000  (8)

Object code:

0000000000001010
0001000000001001
0000000000001011
0111000000001001
1001000000001000
1110000000001001
0100000000001001
1000000000000010
1111000000000000
0000000000000000
0000000000000001
0000000000001010