Register Windows

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Register Windows

• save and restore
• Window over/under flow

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Global Variables

• Use data definition pseudo-ops

.data ! Start of data
segment x .word 12 ! int x 12 y .byte
'a' ! char y 'a' .align 2 ! Get alignment
correct z .half 0xffff ! short z
0xffff .align 4 ! Get alignment
correct lst .skip 4 100 ! int
lst100 .text ! Start of program
segment . . .
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Addressing Global Data
x .word 3 . . . mov x, l0 ld
l0, o1
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How do we handle addresses?

• Use sethi to set high 22 bits
• Use or to set low 10 bits

x .word 3 . . . sethi x gtgt 10, l0
or l0, x 0x3ff, l0 ld l0,
o1
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Alternatives
x .word 3 . . . sethi hi(x), l0
or l0, lo(x), l0 ld l0, o1
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6. Subroutines and Parameters

• Simple Call

call label
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Return
ret
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Skeleton for Subroutines
call SUBR nop ! Delay
slot ... SUBR save sp, ..., sp
... ret restore ! Delay slot
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Parameter Passing

• Registers
• o6 Stack Pointer/Frame Pointer
• o7 Return Address
• o0...o5 Available for six parameters

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Stack Space

• Space on the stack must be reserved for
• Window overflow (64 bytes)
• A structure pointer (4 bytes)
• Register parameters (24 bytes)
• Local variables (? bytes)

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More Than Six Parameters?

• Use the stack

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Return Values

• i0 for word values
• Pointer to structure on stack for larger
  structures

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Leaf Subroutines

• No save or restore
• Use only out registers
• Return

retl
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Separate Assembly Compilation

• C and Assembly Language
• Must have a .global directive  

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Example C Code
/ Demonstrate linking C with assembly. George
Wells - 15 July 1992. Original program by R.
Paul. / void summer (int acc, char
ptr) main (int argc, char argv ) int
sum 0 while (--argc) summer(sum,
argv) printf("sum is d\n", sum) /
main /
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Example Assembler
include(macro_defs.m) ! Define symbolic constants
for parameters. define(acc, i0) !
Pointer to sum define(ptr, i1) !
Pointer to string begin_fn(summer)! void
summer(intacc, charptr) call atoi
! get atoi(ptr) mov ptr, o0 !
parameter to atoi ld acc,
o1 add o0, o1, o0 ! acc
atoi(ptr) st o0, acc end_fn
! summer
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Makefile
sum1 sum1.o summer.o gcc -g sum1.o
summer.o -o sum1 sum1.o sum1.c gcc -g
-c sum1.c summer.o summer.s gcc -g -c
summer.s -o summer.o summer.s summer.m
rm -f summer.s m4 summer.m gt summer.s
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Calling C Routines

• Very simple

fmt .asciz "The answer is d\n" . . .
set fmt, o0 ! fmt - 1st parameter call
printf ! printf("The ...", ans) mov
l3, o1 ! ans as 2nd parameter
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