Overview

1
Overview

• Projects
• The Assembly Process
• Programmed I/O
• Interrupt Driven I/O

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LC-3 Assembly Language
Syntax
LABEL OPCODE OPERANDS COMMENTS
optional
mandatory
Symbol Table
Symbol Address Start x3000 Test
x3005 Next x300A
Number x3010 String x3011
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An Assembly Language Program

• Program to multiply a number by the constant 6
• .ORIG x3050
• LD R1, SIX
• LD R2, NUMBER
• AND R3, R3, 0 Clear R3. It will
• contain the product.
• The inner loop
• AGAIN ADD R3, R3, R2
• ADD R1, R1, -1 R1 keeps track of
• BRp AGAIN the iteration.
• HALT
• NUMBER .BLKW 1
• SIX .FILL x0006

Symbol Table Symbol Address

AGAIN x3053
NUMBER x3057
SIX x3058
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Count the occurrences of a character in a file (1
0f 2).

• Program to count occurrences of a character in
  a file.
• Character to be input from the keyboard.
• Result to be displayed on the monitor.
• Program only works if no more than 9
  occurrences are found.
• Initialization
• .ORIG x3000
• AND R2, R2, 0 R2 is counter, initially 0
• LD R3, PTR R3 is pointer to character file
• GETC R0 gets input character
• LDR R1, R3, 0 R1 gets first character from
  file
• Test character for end of file
• TEST ADD R4, R1, -4 Test for EOT (ASCII x04)
• BRz OUTPUT If done, prepare the output

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Count the occurrences of a character in a file (2
of 2).

• Output the count.
• OUTPUT LD R0, ASCII Load the ASCII template
• ADD R0, R0, R2 Covert binary count to ASCII
• OUT ASCII code in R0 is displayed.
• HALT Halt machine
• Storage for pointer and ASCII template
• ASCII .FILL x0030 ASCII offset
• PTR .FILL x4000 PTR to character file
• .END

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One Pass vs Two Pass Assemblers

• Two Pass Checks for syntax errors and builds
  the Symbol Table during first pass, resolves
  operand addresses during second pass.
• One Pass Checks for syntax errors, builds the
  Symbol Table, and resolves operand addresses
  during the first pass. So why have a two pass?

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More than One Object (Load) File

• Symbol Table
• Symbols Externals Exports Addresses
• Start
  x3000
• Number
  x300A
• Data ?
• Value ?

• The Linker/Loader would generate another
  global table to resolve
• Externals Exports at Load time

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Input / Output
Memory Mapped I/O A section of the memory
address space is reserved for I/O Registers
rather than general memory locations. Think of
it as pseudo memory. The same instructions are
used for general programming and I/O
programming. Non-Memory Mapped I/O There is a
separate address space for I/O programming, and
an entirely separate set of I/O Instructions.
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LC-3 has Memory Mapped I/O
LC-3 Memory Layout x0000 x00FF Trap
vectors x0100 x2FFF System Programs
Data x3000 xFDFF User Programs
Area xFE00 xFFFF I/O Programming
Registers
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Synchronous vs Asynchronous I/O
Synchronous data could be expected to be
available when the program wanted it like data
in memory. This is not realistic for I/O
Asynchronous computer is generally much
faster than I/O so program must wait until data
is available or data has been taken.
Handshaking is used to ensure that data is
available or I/O device is ready
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Polling vs Interrrupt Driven I/O
Polling program checks handshaking signals to
find when data is available of device is done
(typically a loop in the program) Interrupt
program initiates I/O and waits until data is
available (typically goes to sleep until the
operating system wakes the program up)
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Keyboard I/O
KBDR Assigned to xFE02 Data is in
KBDR70 Read only Register KBSR
Assigned to xFE00 Status is in
KBSR15 Set to 1 when new
data is ready Cleared when
data is read
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Simple Input from Keyboard
START LDI R1, A Test for
BRzp
START character input LDI
R0, B BRnzp NEXT_TASK
Go to the next task A .FILL xFE00
Address of KBSR B .FILL
xFE02 Address of KBDR
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Monitor I/O
DDR Assigned to xFE06 Data is
in DDR70 DSR Assigned to xFE04
Status is in DSR15 Set
to 1 when data is picked up
Cleared when new data is written
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Simple Ouput to Monitor
START LDI R1, A Test to see
if BRzp START output
register is ready STI R0,
B BRnzp NEXT_TASK A
.FILL xFE04 Address of DSR B
.FILL xFE06 Address of DDR
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Echo from Keyboard to Monitor
START LDI R1, KBSR Test for
character input BRzp
START LDI R0,
KBDR ECHO LDI R1, DSR
Test output register ready BRzp
ECHO STI R0,
DDR BRnzp NEXT_TASK
KBSR .FILL xFE00
Address of KBSR KBDR .FILL xFE02
Address of KBDR DSR
.FILL xFE04 Address of DSR
DDR .FILL xFE06 Address
of DDR
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The I/O Routine for the LC-3 Keyboard
START ST R1,SaveR1 Save registers
needed ST R2,SaveR2 by this
routine ST R3,SaveR3 LD
R2,Newline L1 LDI R3,DSR BRzp
L1 Loop until Monitor is ready
STI R2,DDR Move cursor to new clean
line LEA R1,Prompt Starting
address of prompt string Loop LDR R0,R1,0
Write the input prompt BRz Input
End of prompt string L2 LDI
R3,DSR BRzp L2 Loop until
Monitor is ready STI R0,DDR
Write next prompt character ADD
R1,R1,1 Increment Prompt pointer
BRnzp Loop Get next prompt
character Input LDI R3,KBSR BRzp
Input Poll until a character is typed
LDI R0,KBDR Load input character
into R0 L3 LDI R3,DSR BRzp L3
Loop until Monitor is ready
STI R0,DDR Echo input character L4
LDI R3,DSR BRzp L4
Loop until Monitor is ready STI
R2,DDR Move cursor to new clean line
LD R1,SaveR1 Restore registers
LD R2,SaveR2 to original values
LD R3,SaveR3 BRnzp NEXT_TASK
Do the program's next task
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The I/O Routine for the LC-3 Keyboard (2)
SaveR1 .BKLW 1 Memory for
registers saved SaveR2 .BKLW 1 SaveR3 .BKLW
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"
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Keyboard I/O
KBDR Assigned to xFE02 Data is in
KBDR70 Read only Register KBSR
Assigned to xFE00 Status is in
KBSR15 Set to 1 when new
data is ready Cleared when
data is read Interrupt Request is
in KBSR14
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Monitor I/O
DDR Assigned to xFE06 Data is
in DDR70 DSR Assigned to xFE04
Status is in DSR15 Set
to 1 when data is picked up
Cleared when new data is written
Interrupt Request is in DRS14