Instructor: Nachiket M' Kharalkar

1
Introduction to Microcontrollers

• Instructor Nachiket M. Kharalkar
•  
• Lecture 7
• Date 06/15/2007
• E-mail knachike_at_ece.utexas.edu
•  

2
Todays Agenda

• Quiz 1 June 18th
• Exam 1 June 25th
• Get multimeter wire stripper
• Page 2 instructions
• Recap
• Conditional branching
• Logical operations
• Shift operations
• Friendly program
• Stack

3
mul an 8-bit by 8-bit into 16-bit unsigned
multiply

• The mul instruction takes two 8-bit inputs and
  generates a 16-bit product.
• Condition code bits are set after RAB.
• C R7, set if bit 7 of 16-bit result is one

4
idiv performs 16-bit by 16-bit unsigned divide

• The idiv instruction takes two 16-bit inputs
  and generates a 16-bit quotient and a 16-bit
  remainder.
• Condition code bits are set after
  quotientdividend/divisor.
• Z result is zero,
• V 0
• C divide by zero,
• Cnot(X15)not(X14)...not(X2)not(X1)not(X0)

5
fdiv performs a 16-bit by 16-bit unsigned divide

• The fdiv instruction takes two 16-bit inputs
  and generates a 16-bit quotient and a 16-bit
  remainder.
• Condition code bits are set after
  R(65536D)/X.
• Z result is zero,
• V overflow if RegX RegD, result gtFFFF
• C divide by zero,

6
Multiply, divide instructions

• emul YD YD unsigned multiply
• emuls YD YD signed multiply
• ediv Y YD/X unsigned divide (D Reminder)
• edivs Y YD/X signed divide (D Reminder)

7
Signed branches, branch if

• bge place greater than or equal to
• if (NV)0
• i.e., (NVNV)0
• bgt place greater than
• if (ZNV)0
• i.e., (ZNVNV)0
• ble place less than or equal to
• if (ZNV)1
• i.e., (ZNVNV)1
• blt place less than
• if (NV)1
• i.e., (NVNV)1

8
Unsigned branches, branch if

• bhs place greater than or equal to
• if C0, same as bcc
• bhi place greater than
• if CZ0
• blo place less than
• if C1, same as bcs
• bls place less than or equal to
• if CZ1

9

• It is important to know
• precision (e.g., 8-bit, 16-bit)
• format (e.g., unsigned, signed)
• It takes three steps
• read the first value into a register
• compare the first value with the second value
• conditional branch
• When testing for equal or not equal
• doesnt matter whether signed or unsigned
• still matters if 8-bit or 16-bit
• doesnt matter about load and compare order

10
8-bit if-then compare to zero examples
Assume G is an 8-bit global variable, signed or
unsigned
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8-bit if-then signed compare to zero examples
Assume sG is a signed 8-bit global variable
12
8-bit if-then compare examples
Assume G1 G2 are 8-bit global variables, signed
or unsigned
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Compare 8-bit versus 16-bit condition instructions
Assume G1 G2 are 8-bit global variables, signed
or unsigned Assume H1 H2 are 16-bit global
variables, signed or unsigned
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Compare the four possible inequalities
Assume uG is an unsigned 8-bit global variable
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for(uG0uGlt5uG) clr
uG loop  ldaa uG       cmpa 5       bhs  next  s
top when uGgt5      ????? body of while
loop inc uG       bra  loop next for(i
0ilt5i) // something 5 times ldaa 5
loop 5 down to 0 loop  ????? body of for
loop dbne A,loop
16

• do
• ?????
• while(uG lt 5)
• loop  ????? body of while loop
• ldaa uG
•       cmpa 5
•       blo  loop  stop when uGgt5

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Assembly directives supported by TExaS
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• Form double byte
• (ltlabelgt) fdb ltexprgt(,ltexprgt,...,ltexprgt)
• (ltlabelgt) dc.w ltexprgt(,ltexprgt,...,ltexprgt)
• (ltlabelgt) dw ltexprgt(,ltexprgt,...,ltexprgt)
• (ltlabelgt) .word ltexprgt(,ltexprgt,...,ltexprgt)
• org FFFE
• fdb main
• Define 32-bit constant
• (ltlabelgt) dc.l ltexprgt(,ltexprgt,...,ltexprgt)
• (ltlabelgt) dl ltexprgt(,ltexprgt,...,ltexprgt)
• (ltlabelgt) .long ltexprgt(,ltexprgt,...,ltexprgt)
• S1  dl     100000,12345678
• S2  .long  1,1000,1000000,1000000000
• S3  dc.l   -1,0,1

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Sample program

• org 3800
• size equ 5
• data rmb size
• org 4000
• sum ldaa size
• ldx data
• clrb
• loop addb 1,x
• dbne A,loop
• org FFFE
• fdb sum

20
Logical operations

• The N bit will be set is the result is negative.
• The Z bit will be set if the result is zero.
• Clear V0 bit.
• anda w RegARegAw
• anda U RegARegAU
• andb w RegBRegBw
• andb U RegBRegBU
• bita w RegAw
• bita U RegAU
• bitb w RegBw
• bitb U RegBU
• coma RegAFF-RegA, RegARegA
• comb RegBFF-RegB, RegBRegB
• eora w RegARegA w
• eora U RegARegA U
• eorb w RegBRegB w
• eorb U RegBRegB U
• oraa w RegARegA w
• oraa U RegARegA U

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Friendly software

• Set bit 2 of port T
• ldaa 00000100
• oraa PTT
• staa PTT
• Clear bit 5 of port AD
• ldaa 11011111
• anda PTAD
• staa PTAD

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Shift operations

• The N bit is set if the result is negative.
• The Z bit is set if the result is zero.
• The V bit is set on a signed overflow, change in
  the sign bit.
• The C bit is the carry out after the shift.
• asla RegARegA2 (same as lsla)
• aslb RegBRegB2 (same as lslb)
• asld RegDRegD2 (same as lsld)
• lsla RegARegA2 (same as asla)
• lslb RegBRegB2 (same as aslb)
• lsld RegDRegD2 (same as asld)
• asra RegARegA/2 (signed)
• asrb RegBRegB/2 (signed)
• asrd RegDRegD/2 (signed)
• lsra RegARegA/2 (unsigned)
• lsrb RegBRegB/2 (unsigned)
• lsrd RegDRegD/2 (unsigned)
• rola RegArol(RegA) (C?A7??A0?C)
• rolb RegBrol(RegB) (C?B7??B0?C)
• rora RegAror(RegA) (C?A7??A0?C)

23
8-bit logical shift right
8-bit arithmetic shift right.
8-bit shift left
8-bit roll right and 8-bit roll left
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Subroutines and the stack

• classical definition of the stack
• push saves data on the top of the stack,
• pull removes data from the top of the stack
• stack implements last in first out (LIFO)
  behavior
• stack pointer (SP) points to top element
• many uses of the stack
• temporary calculations
• subroutine (function) return addresses
• subroutine (function) parameters
• local variables

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ldaa 1 psha
lds 4000
ldaa 2 psha
ldaa 3 psha
pulb
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• The push and pull instructions
• psha push Register A on the stack
• pshb push Register B on the stack
• pshx push Register X on the stack
• pshy push Register Y on the stack
• des S S-1 (reserve space)
• pula pull from stack into A
• pulb pull from stack into B
• pulx pull from stack into X
• puly pull from stack into Y
• ins SS1 (discard top of stack)
• The following are important transfer instructions
• tsx transfer S to X
• tsy transfer S to Y
• txs transfer X to S

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We use the term subroutine all functions or
procedures

• whether or not they return a value
• develop modular software
• called by either bsr or jsr
• subroutine returns using rts

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• org 4000
• main lds 4000 initialize stack
• clra
• loop bsr sub branch to subroutine
• bra loop
• Purpose increment a number
• Input RegA, range 0 to 255
• Output RegAInput1
• Errors Will overflow if input is 255
• sub inca adds one to Input
• rts
• org fffe
• fdb main

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Execution of the bsr instruction

• Opcode fetch R 0x4004 0x07 from EEPROM
• Operand fetch R 0x4005 0x02 from EEPROM
• Stack store lsbW 0x3FFF 0x06 to RAM
• Stack store msbW 0x3FFE 0x40 to RAM

30
The stack before and after execution of the bsr
instruction
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Execution of the rts instruction

• Opcode fetch R 0x4009 0x3D from EEPROM
• Stack read msb R 0x3FFE 0x40 from RAM
• Stack read lsb R 0x3FFF 0x06 from RAM

32
The stack before and after execution of the rts
instruction