Chapter 9 Overview

1
Chapter 9Overview

• Traps mechanism RET
• Subroutines JSR JSRR RET
• Interrupt mechanism RTI

2
LC-3 has Memory Mapped I/O
LC-3 Memory Layout x0000 x00FF Trap
vectors (Supports Software Interrupts)
x0020 x0400 GETC (Read Char from
Keyboard) x0021 x0430 OUT
(Write Character to Console) x0022
x0450 PUTS (Write string to Console)
x0023 x04A0 IN (Prompt,
input character from Keyboard, echo character to
Console) x0024 x04E0 PUTSP
(Write packed string to Console)
x0025 xFD70 HALT (Turn off run latch in
MCR) x0100 x01FF Interrupt Vectors
(Supports Hardware Interrupts) x0200 x2FFF
System Programs Data (Operating System)
x3000 xFDFF User Programs Area xFE00
xFFFF I/O Programming Registers (Mapped I/O
Registers) xFE00 KBSR 15 Ready, 14
Intr enable (Keyboard Status Register)
xFE02 KBDR 70ascii data
(Keyboard Data Register) xFE04
DSR 15Done, 14Intr enable
(Display Status Register) xFE06 DDR
70ascii data
(Display Data Register xFFFE MCR
15Run latch
(Machine Control Register)
3
Trap Routine
Trap Instruction TRAP x 1111 0000 trap
vector F0xx PC ? R7 Jump to routine at
trap vector address Return RET 1100 000 111
000000 C1C0 R7 ? PC
(JMP R7)
4
Traps

• Execute TRAP vector - Operating System
  Service Routines
• 2) Trap Vectors are at memory locations
  000000FF
• Trap Vectors contain addresses of Trap Service
  Routines
• PC is stored in R7
• Address of Trap Service Routine loaded into PC
• Service Routine Program executed
• Trap service routine program ends with an RET
• ( R7 loaded into PC)

5
TRAP x21 OUT Trap Vector Routine (Output
Character) out.asm .ORIG x0430
System call starting address ST
R1, SaveR1 R1 will be used to poll the DSR
hardware Write
the character TryWrite LDI R1, DSR
Get status BRzp TryWrite Bit
15 on says display is ready WriteIt STI R0,
DDR Write character return from
trap Return LD R1, SaveR1 Restore
registers RET Return from trap
(JMP R7, actually) DSR .FILL xFE04
Address of display status register DDR
.FILL xFE06 Address of display data
register SaveR1 .BLKW 1 .END
6
TRAP x23 IN Trap Service Routine
(Input Character) Service
Routine for Keyboard Input .ORIG
x04A0 START ST R1,SaveR1 Save the
values in the registers ST R2,SaveR2
that are used so that they ST
R3,SaveR3 can be restored before RET
LD R2,Newline L1 LDI R3,DSR
Check DDR -- is it free? BRzp
L1 STI R2,DDR Move cursor
to new clean line LEA R1,Prompt
Prompt is starting address
of prompt string Loop LDR
R0,R1,0 Get next prompt character
BRz Input Check for end of
prompt string L2 LDI R3,DSR BRzp
L2 STI R0,DDR Write
next character of
prompt string ADD R1,R1,1
Increment Prompt pointer BRnzp
Loop Input LDI R3,KBSR Has a
character been typed? BRzp Input
LDI R0,KBDR Load it into R0 L3
LDI R3,DSR BRzp L3 STI
R0,DDR Echo input character
to the monitor L4
LDI R3,DSR BRzp L4 STI
R2,DDR Move cursor to new clean line
LD R1,SaveR1 Service routine
done, restore LD R2,SaveR2
original values in registers. LD
R3,SaveR3 RET
Return from trap (i.e., JMP R7) SaveR1 .BLKW
1 SaveR2 .BLKW 1 SaveR3 .BLKW 1 DSR
.FILL xFE04 DDR .FILL xFE06 KBSR .FILL
xFE00 KBDR .FILL xFE02 Newline .FILL
x000A ASCII code for newline Prompt
.STRINGZ "Input a charactergt" .END
7
TRAP x25 HALT Service Routine
.ORIG xFD70 Where this
routine resides ST R7,
SaveR7 ST R1, SaveR1
R1 a temp for MC register ST
R0, SaveR0 R0 is used as working space
print message that machine is halting
LD R0, ASCIINewLine
TRAP x21 LEA R0, Message
TRAP x22 LD
R0, ASCIINewLine TRAP x21
clear bit 15 at xFFFE to stop the machine
LDI R1, MCR Load MC
register into R1 LD R0, MASK
R0 x7FFF AND R0,
R1, R0 Mask to clear the top bit
STI R0, MCR Store R0 into MC
register return from HALT routine. (how
can this routine return if the machine is halted
above?) LD R1, SaveR1
Restore registers LD R0,
SaveR0 LD R7, SaveR7
RET JMP R7,
actually Some constants ASCIINewLine
.FILL x000A SaveR0 .BLKW 1 SaveR1
.BLKW 1 SaveR7 .BLKW 1 Message
.STRINGZ "Halting the machine." MCR
.FILL xFFFE Address of MCR MASK
.FILL x7FFF Mask to clear the top
bit .END
8
TRAP 22 PUTS Trap Service Routine (Output a
Character String) puts.asm This service
routine writes a NULL-terminated string to the
console. It services the PUTS service call
(TRAP x22). Inputs R0 is a pointer to the
string to print. Context Information R0, R1,
and R3 are saved, and R7 is lost in the
jump to this routine .ORIG x0450
Where this ISR resides
ST R7, SaveR7 Save R7 for later
return ST R0, SaveR0
Save other registers that ST
R1, SaveR1 Are needed by this routine
ST R3, SaveR3 Loop
through each character in the array Loop
LDR R1, R0, 0 Retrieve the
character(s) BRz Return
If it is 0, done L2 LDI R3,DSR
BRzp L2 STI R1, DDR
Write the character ADD
R0, R0, 1 Increment pointer
BRnzp Loop Do it all over
again Return from the request for service
call Return LD R3, SaveR3
LD R1, SaveR1 LD R0,
SaveR0 LD R7, SaveR7
RET Register locations DSR .FILL
xFE04 DDR .FILL xFE06 SaveR0
.FILL x0000 SaveR1 .FILL x0000 SaveR3
.FILL x0000 SaveR7 .FILL x0000
.END
9

• Programming Exercise 1
• Write a program to add the contents of R0 and R1,
  and indicate in R2 if there was an overflow

10
Programming Exercise 1

• Add R3R0R1, R20 indicates no overflow
• .ORIG x3000
• AND R2, R2, 0 Initially R20 (no Overflow
  assumed)
• ADD R3, R0, R1 R3R0R1
• test for overflow
• ADD R0, R0, 0 test R0
• BRN NEG Branch if RO negative
• ADD R1, R1, 0 test R1
• BRN DONE No overflow if operand signs
  differ (R1 NEG)
• ADD R3, R3, 0 maybe, test R3
• BRZP DONE No overflow if result sign
  matches (All POS)
• ADD R2, R2, 1 R21 indicating overflow
• NEG ADD R1, R1, 0 test R1
• BRZP DONE No overflow if operand signs
  differ (R1 POS)

11

• Programming Exercise 2
• Write a program to count the 1s in register R0

12
Programming Exercise 2

• Program to count 1's in Register R0
• R3 is a working copy of R0
• R1 contains the count
• R2 is a loop counter
• .orig x3100
• ADD R3, R0, 0 copy R0 into R3
• AND R1, R1, 0 clear count
• ADD R3, R3, 0 test for Neg
• BRZP NEXT count if Neg
• ADD R1, R1, 1
• NEXT AND R2, R2, 0 check remaining 15 bits
• ADD R2, R2, -15
• LOOP ADD R3, R3, R3 shift R3 left
• BRZP AGAIN count if Neg
• ADD R1, R1, 1
• AGAIN ADD R2, R2, 1 loop until done
• BRN LOOP

13

• Programming Exercise 3
• Write a program to add two, two digit numbers
  read from the console

14

• Programming Exercise 3

Program to add two 2 digit decimal numbers read
from the console R1 R2 are working
registers to load 2 digit number R3 is first
number R4 is second number R5 is the
sum R6 is conversion offset .orig
x3600 LEA R0, MESSAGE print message
PUTS Get first number LD R0, NEWLINE
print PROMPT1 OUT OUT LEA
R0, PROMPT1 PUTS GETC get
first character OUT LD R6, M30
convert char to hex ADD R0, R0, R6
ADD R1, R0, R0 R1 2xR0 ADD R2, R1,
0 copy R1 into R2 ADD R2, R2, R2 R2
4xR0 ADD R2, R2, R2 R2 8xR0
ADD R2, R2, R1 R2 10xR0 GETC
get second character OUT ADD R0,
R0, R6 convert to hex ADD R3, R2, R0
R3 first decimal number Get second number
LEA R0, PROMPT2 get first character
PUTS GETC OUT ADD R0, R0, R6
convert char to hex ADD R1, R0, R0
R1 2xR0 ADD R2, R1, 0 copy R1 into
R2 ADD R2, R2, R2 R2 4xR0 ADD
R2, R2, R2 R2 8xR0 ADD R2, R2, R1
R2 10xR0 GETC get second
character OUT ADD R0, R0, R6
convert to hex ADD R4, R2, R0 R4
first decimal number
15

• Programming Exercise 3 (2)

Add the numbers and print results ADD
R5, R4, R3 R5 R3 R4 LEA R0, SUM
prepare to print results PUTS
LD R4, P100 find 1st digit LD R3,
M100 AND R0, R0, 0 LOOP1 ADD R0, R0, 1
ADD R5, R5, R3 subtract 100 until
negative BRZP LOOP1 ADD R5, R5, R4
ADD R0, R0, -1 LD R6, P30
convert to ascii print ADD R0, R0, R6
OUT AND R0, R0, 0 find 2nd
digit LOOP2 ADD R0, R0, 1 ADD R5, R5,
-10 subtract 10 until negative BRZP
LOOP2 ADD R5, R5, 10 ADD R0, R0,
-1 LD R6, P30 convert to ascii
print ADD R0, R0, R6 OUT
ADD R0, R5, R6 convert and print 3rd digit
OUT LD R0, NEWLINE OUT
HALT MESSAGE .STRINGZ "Enter two 2-digit decimal
numbers" NEWLINE .FILL x000A PROMPT1 .STRINGZ "
The sum of " PROMPT2 .STRINGZ " and " SUM
.STRINGZ " is " M30 .FILL xFFD0 -x30 P30
.FILL X0030 x30 M100 .FILL xFF9C
-100 P100 .FILL x0064 100 .END