Reptile: Example Programs

Reptile has a minimum instruction set which is capable of solving quite complicated problems.. In the coming lectures we will continue to add a few more instructions like PUSH, POP, CALL and RET but it is quite sufficient even without these instructions.. Below we will give examples of the sort of problems which are solvable by reptile

Note that these programs can be written in many different ways..

A note on jumps

Note that we only have jz as a conditional jump, ie, the jump will occur when the result of the prior ALU operation is zero. In programming situations we will require more, ie, we want instructions like

jgt r1, r2, xyz  // r1, r2 denotes two registers 
                 //and xyz denotes an address/label

Which will jump to address xyz when r1>r2 and will execute the next instruction otherwise. But it is possible to construct a small code segment which is functionally the same with jgt instruction:

sub r3 r1 r2   //twos complement subtraction
ldi r4, 0x8000
and r3 r3 r4   //check the sign bit
jz  xyz        //jump if sign bit is zero, ie, if r1>r2

jump greater or equal instruction, jge r1, r2, xyz, can be “simulated in a similar way

sub r3 r1 r2 //twos complement subtraction 
jz  xyz     //jump if r1 =r2
ldi r4, 0x8000 
and r3 r3 r4 //check the sign bit 
jz xyz //jump if sign bit is zero, ie, if r1>r2

Summing all integers from 1 to N

The program has two variables, N and SUM. N is considered unsigned.
The program will sum all the integers between 1 and N
The program will write its result to the variable SUM.

In the following program, N is arbitrarily chosen as 60.

.data
           N: 60
           sum:  
.code 
           ldi 0 N
           ld 0 0
           ldi 3 0x0
           or  3 0 3   
           jz out       //checks if N=0
loop       inc 1 
           add 2 2 1           
           dec 0 
           jz out 
           jmp loop 
out        ld 0 sum
           st 0 2
end        jmp end

Summing an alternating series

Same with the above, but this time evaluates 1-2+3-4+5-6+7-8+…. up to N.

.data
           N: 60
           sum:  
.code 
           ldi 0 N 
           ld  0 0 
           ldi 3 0x0
           or 3 0 3 jz out //checks if N=0
loop       inc 1 
           add 2 2 1           
           dec 0 
           jz out 
           inc 1
           sub 2 2 1
           dec 0
           jz out
           jmp loop 
out        ld 0 sum
           st 0 2
end        jmp end

Multiplying unsigned integers A and B

.data
           A:20
           B:30           
           mult:  
.code 
           ldi 0 A 
           ld  0 0 
           ldi 1 B
           ld 1 1
           ldi 3 0x0
           or 3 1 3 
           jz out //checks if B==0       
loop       add 2 2 0   
           dec 1           
           jz out
           jmp loop
           ld 0 mult
           st 0 2
end        jmp end

Reptile: Example Programs