HW

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HW 1 Problems Industrial Strength Assembly

• CSCI 136

2
Reading the Question

• If there are no bs in the string, then bfind
  should return a pointer to the null character at
  the end of the string
• lbu t2, 0(t3)
• beq t2, 0, term
• term la a0, ndone
• li v0 ,4
• syscall
• Its great that you tell me whats happening.. How
  about putting t3 in to v0?

3
Reading the Question

• The bfind procedure should locate the first b
  character in the string and return its address in
  register v0
• bfind lbu t1,0(a0)
• beqz t1, Ret
• beq t1, b, Ret
• addi a0, a0, 1
• j bfind
• Ret sb v0, (t1)

4
Using the bfind procedure in bcount

• You must use your bfind procedure in Exercise
  3.23 in your implementation of bcount.
• What does bfind return?
• So how do we check if were at the end of the
  string?
• jal bfind
• lb t0,0(v0)
• beqz t0, endOfString

5
RTFQ Reading the Question

• Tough parts of Software
• Specifying Requirements
• Understanding the Specified Requirements
• Why it matters at GWU Grading Scripts
• Besides.. It should be really frustrating to
  spend hours answering the wrong question

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RTFQ- Reading the Question

• Typical Errors
• Didnt write bcount as a procedure
• This requires saving ra before the call to
  bfind, and restoring ra after the call to bfind
• main
• jal bcount
• bcount jal bfind
• What happens to ra when jal is called?

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Answering the Question

• add t0, 0, 0
• bfind add t3, t0, a0
• lbu t2, 0 (t3)
• beq t2, 0, done
• addi t2, t2, -98
• beq t2, 0, done
• addi t0, t0, 1
• j bfind
• done addi t0,1
• add v0,t0,0
• jr ra

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Answering the Question

• li t0,0
• bfind add t3,a0,t0
• lbu t2, 0 (t3)
• beq t2, 0, done
• beq t2, b, done
• addi t0, t0, 4
• j bfind
• done move s2,t0
• jr ra

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Why do we program in Assembly?
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Why do we program in Assembly?

• What matters in real world
• SPEED, SPEED SPEED
• Other acceptable answer
• Low Level Interface with hardware (mouse,
  ethernet card, serial port, video card, etc.)
• Unfortunately we cant simulate these hardware
  interfaces with SPIM.

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Naïve Approach to Style

• Good Style lots of comments
• Even poor code well
• documented is still
• poor code.

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Style

• Accomplish task in straight forward manner with
  minimum effort
• Unfortunately, Good Style comes from experience
  and experience comes from Bad Style
• We dont expect you to have wonderful style in
  the first couple assignments
• By the end of semester you should be writing
  simple straight forward code.

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Minimum Effort

• la t0, a0
• bfind lbu t1, 0 (t0)
• beq t1, Done
• beq t1,b, Done
• addi t0, 1
• addi a0, 1
• j bfind
• Done
• move v0,a0

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Minimum Effort

• Registers are a Commodity. Do not waste them!
• Why is minimizing the use of registers so
  important?
• What is the slowest type of instruction on our
  processor?
• If we have registers available... We can use
  them.
• This will allow your programs to run faster!

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Minimum Effort

• Registers are a Commodity. Do not waste them!
• Why is minimizing the use of registers so
  important?
• Modern processors have multiple execution units
• Processors can execute several different portions
  of your code at the same time...
• Lack of registers prevents multiple execution
• Help the processor out by using minimal number of
  registers and having minimal register overlap.
• This will allow your programs to run faster!

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Do we need all these registers?

• li s1,0x00
• bfind
• lbu t2,0(a0)
• beq t2,s1,exit
• beq t2,b,exit
• addi a0,a0,1
• j bfind
• exit

17
Do we need all this code?

• bfind
• lbu t2,0(a0)
• addi a0,a0,1
• beq t2,zero,exit
• beq t2,b,exit
• j bfind
• exit
• subu a0,a0,1

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Minimum Effort

• Know your architecture (load - store) and what
  your pseudo instructions do
• Is there a difference difference between
• beq t0, b, Done
• and
• addi at,zero,b
• beq t0,at, Done

19
Do I need everything in my loop?

• bfind lbu t0, 0 (a0)
• beqz t0, done
• addi t1,zero,b
• beq t0, t1, done
• add a0, a0, 1
• j bfind
• done mov v0, a0
• jr ra

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Extra Branches Border Case Failure

• bfind li t1, b
• lb t0,0(a0)
• bne t0,t1,loop
• move v0,a0
• quit jr ra
• loop addi a0,a0,1
• lbu t0,0(a0)
• beq t0,zero,quit
• bne t0,t1,loop
• move v0,a0
• jr ra

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Extra Branches and Loops

• Why are extra branches bad?
• -Optimizing Branches
• Why are extra loops bad?

22
Public and Private Variables

• In SPIM- When you call a function and the
  function returns the only values from that
  function your procedure should access are
• v0, v1
• Values saved by the function on the stack
• Locations in Memory
• DO NOT access the internal variables (registers)
  of another function! This breaks the whole
  function paradigm. It isnt a black box if
  you go digging around

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Public and Private Variables
GWU
FUNCTION
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Breaking the Function Paradigm

• addi sp, sp, -4
• sw ra,0(sp)
• jal bfind
• lw ra,0(sp)
• addi sp,sp,4
• quit bne t0, zero, adder
• Who set t0? bfind did. But the only return
  value was supposed to be v0?
• What are the problems with this?

25
Breaking the Function Paradigm

• I should be able to plug in ANY bfind in to
  bcount and make it work.
• Dont fight the framework!

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Real World Disclaimer

• Register assignments arent as structured
• (no t0-t9, s0-s7..just registers 1-31)
• Saving values on stack or in memory is slow.
• More registers can be used to return values.
• Function writers and function callers typically
  have specific agreements about what registers to
  use for passing and returning values.

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Proper Instructions

• jr 1 is jump to a value given by the register 1
  (Where do you typically see jr ra ?)
• j is jump to a label
• Use addi to add a constant to a register
• Use add to add registers together
• Watch the ordering of registers in instruction
• sw sp, ra What does this do?

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Good Implementation of Bfind

• addi t1,zero,b
• bfind lbu t0, 0 (a0)
• beqz t0, done
• beq t0, t1, done
• add a0, a0, 1
• j bfind
• done move v0, a0
• jr ra
• 8 lines of assembly. No extra branches, loops,
  variables. Common case is fast.

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Lab Assignment Endian Fixer

• TCP/IP message headers are in Big Endian
• Your PC is a Little Endian machine
• Write function that takes word value in a0 and
  returns a Endian reversed value in v0
• Goals
• Learn how to use masks
• li t0, 0x000000ff
• andi t1, a0, t0
• ori v0, t0, t1

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• endianFixer 1234
• rol a0,a0,8 2341
• lui t1, 0x00ff
• ori t1, t1, 0x00ff
• and v0,t1,a0 0301
• sll t1,t1,8 t1ff00ff00
• rol a0,a0,16 4123
• and a0,t1,a0 4020
• or v0,v0,a0 4321
• jr ra

Why dont I ever zeroize v0? Is li
t1,0x00ff00ff ok?
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Adder Review

• How many delays does a 4 bit ripple carry adder
  take to add two 4 bit numbers? Why?

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Adder Review

• What are the equations for a 4 bit carry look
  ahead adder? Do you understand this?

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• From when a and bs show up how long does it
  take until sum appears on the output lines?
• How long for carry generation how long for sum?

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8 bit adder

• We need an 8 bit adderhow can we do this?

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8 bit adder

• We could have a pure ripple carry
• How long would this take to calculate
• the sums?

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8 bit adder

• We could use ripple carry to connect
• two 4 bit carry look ahead groups
• How long would this take to calculate
• the carries? The sums?

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2nd layer of Carry Lookahead

• Lets add another layer of Carry Lookahead for our
  8 bit adder
• What are the equations for big Ps?
• What are the equations for big Gs?
• What are the equations for the big Cs?

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2nd layer of Carry Lookahead

• How long would it take to calculate the carries?
• How long would it take to calculate the sums?