cosc 2150

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cosc 2150

• Subroutines in ARC

2
Assembly code

• There are two instructions used for subroutine
  calls
• call label
• This will change the PC to where the labels
  address and put the current address into r15
• jmpl rX4, rY
• will jump to the address rX and put the current
  address in rY
• Normally rX is r15, the 4 is because you would
  return the call instruction, instead of the next
  instruction.

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example

• . (instructions)
• call sub_r ! call subroutine sub_r
• sub r2, 2,r1
• . (more instructions)
• halt
• sub_r addcc r1, 2, r2
• jmpl r154, r0 !go back to sub instruction

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Code style

• Subroutines are written so that you execute a
  piece of code and then return back.
• You can not branch out of a subroutine back to
  the "main" code.
• You subroutines are not be "inside" the main code
  either.
• The jmpl command, places the address of the
  instruction in r15. Because we do not have a
  stack for the emulator, you MUST store this
  value.
• The reason will shown in example 2.

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Example 1

• main ()
• int a 2
• a a 3
• sub_r()
• void sub_r()
• a a 1

• .begin
• .org 2048
• ! main ld a,r1add r1,3, r1st r1,
  acall sub_r !call sub sub_rhalt
• ! subroutine sub_r
• sub_r st r15, sub_r_l
• ld a,r1add r1,1,r1st r1, a
• ld sub_r_l, r15jmpl r154,r5
• sub_r_l 0
• a2
• .end

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Example 2

• main ()
• int a2
• sub1()
• sub2()
• sub1()
• sub1 ()
• a a 1
• sub2()
• sub2 ()
• a a 2

• .begin
• .org 2048
• ld a, r1 !load A into r1call sub1 !call
  sub sub1call sub2 !call sub sub2
• call sub1 !call sub sub1halt
• sub1 st r15, sub1_ladd r1, 1, r1st r1,
  acall sub2ld sub1_l, r15jmpl r154, r2
• sub2 st r15, sub2_ladd r1, 2, r1st r1,
  ald sub2_l, r15jmpl r154, r2
• sub2_l 0
• sub1_l 0
• a 2
• .end

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Recursion

• Is recursion possible in ARC code?
• No
• A new subroutine would have to created for each
  instance of the recursion and ARC code does not
  have that possibility.
• because we have store register 15, which would
  over write the old value.

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Example 3

• main ()
• int a1, b 2
• a a b
• sub1()
• b b a
• sub1 ()
• a b 1
• b a 2

• .begin
• .org 2048
• ! main ld a, r1ld b, r2add r1,r2,
  r1st r1, acall sub1 !call subroutine
  sub1add r1, r2, r2
• st r2, b
• halt
• ! subroutine sub1
• sub1 st r15, sub1_lsub r2,1,r1st r1,
  aadd r1, 2, r2st r2, bld sub1_l,
  r15jmpl r154,r5
• sub1_l 0
• a 1
• b 2
• .end

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Example 4

• main()
• int a3, i
• for (i0 ilt3 i)
• if (i gt0)
• sub1()
• else
• ai i 1
• sub1 ()
• ai i - 1

• .begin
• .org 2048
• ld i, r1
• top subcc r1, 3, r0 ! ilt3
• bpos done
• !if (igt0)
• subcc r1, 1, r0
• bneg else
• call sub1
• ba incr
• else sll r1, 2, r2 !ai i -1
• addcc r1, 1, r3
• st r3, r2a
• incr add r1, 1, r1 !i
• ba top
• st r1, i
• done halt
• sub1 st r15, sub1_l

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