to/from C LC3
CIT 593
1
Example 1
int main() { int a = 0; int b = 5; int b = 2; int d = 1; a = b + c + d; }
Assume that, a, b, c, d are initialized for you Write the LC3 code for a=b+c+d
Answer on next slide.
2
Solution Example 1
a=b+c +d
R5
LDR LDR LDR ADD ADD STR
R1, R5, #2 R2, R5, #1 R3, R5, #0 R1, R1, R2 R1, R1, R3 R1, R5, #3
;R1 = b ; R2 = c; ; R3 = d ; R1 = b + c ; R1 = R1 + d ; Store a
d= 1 c=2 b=5 a=0
Local variables get put onto stack in the order in which they appear. Since d is last, it goes on top of the stack
3
Example 2
int a[10] int x = 5; . Also assume that array a is initialized Write the LC3 equivalent code for a[x] = a[x + 1]
Answer on next slide.
4
Solution Example 2
a[x] = a[x + 1] LDR R0, R5, #0 ; R0 = x ADD R0, R0, #1 ; R0 = x+1 ADD R1, R5, #1 ; R1 = &a[0] ADD R1, R0, R1 ; R1 =&a[x+1] LDR R2, R1, #0 ; R2 = a[x + 1] LDR R0, R5, #0 ; R0 = x ADD R1, R5, #1 ; R1 = &a[0] ADD R1, R0, R1 ; R1 = &a[x] STR R2, R1, #0 ; a[x] = R2 = a[x+1]
5
R5
x a[0] a[1] a[2] a[3] a[4] a[5] a[6] a[7] a[8] a[9]
Example 3
struct temp{ int lowT; int highT; }; typedef struct temp T; int main( ){ T day; int avg; day.lowT = -1; .(1) day.highT = 15; .(2) .. return 0; }
Write LC3 code for C code shown in lines (1) and (2)
Answer on next slide.
6
Solution to Example day.lowT 3
= -1 & day.highT = 15
R5
AND R0, R0, #0 ADD R0, R0, #-1 AND R1, R1, #0 ADD R1, R1, #15 ADD R2, R5, #1 ;R2 = start of struct STR R0, R2, #0 ;day.lowT = R0 STR R1, R2, #1 ;day.highT = R1
avg day.lowT day.lowT day.highT day.highT
7
Example 4
In main( ) , the local variables are defined in the following order int a; int * b; int c ;
Below is the LC3 code
ADD R0,R5, #2 STR R0, R5, #1 LDR R1, R5, #1 LDR R1, R1, #0 ADD R1, R1, #1 STR R1, R5, #0 The LC3 code is doing something with a, b, c. Translate the LC3 code to C Answer on next slide.
8
Solution to Example 4
Equivalent C code b = &a; c = *b + 1; ADD R0,R5, #2 ; R0 = &a = addr of a STR R0, R5, #1 ; b = &a LDR R1, R5, #1 ; R1 = &a LDR R1, R1, #0 ; R1 = *b i.e. value of a ADD R1, R1, #1 ; R1 ...