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Introduction to SIMPLE
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1. Introduction to SIMPLE A reduced instruction set High Level Language (HLL) 1BA3 – G Lacey – Semester 1 Lecture 4

2. The Simple programming Language • Simple has only 7 statements • rem - the remainder of the line is a comment • input - read from the keyboard • print - write to the terminal • goto - jump to a line number • if/goto - if a condition is true jump to another line • let - evaluate a mathematical expression • end - end of the program • Every line has a line number • Simple only operates on integers • Simple only has lower case • Variable names have a single letter 1BA3 – G Lacey – Semester 1 Lecture 4

3. The Simple programming Language • Variables can only be integers and do not need to be declared before being used • Simple mathematical operators • Addition, subtraction, multiplication, division are +, -, *, / • greater than >, greater than or equal to >= • less than <, less than or equal to <= • equal to ==, not equal to != • assign the value to = • parentheses ( ) • Examples • 10 rem this is a comment • 22 input x • 33 if x == 0 goto 10 • 40 let r = a + b * c / ( a - d ) • 50 print x 1BA3 – G Lacey – Semester 1 Lecture 4

4. read a read b sum = a + b print sum end Determine and print the sum of two numbers 10 rem add and print the sum of 2 nums 20 input a 30 input b 40 let c = a + b 50 print c 60 end 1BA3 – G Lacey – Semester 1 Lecture 4

5. read a read b true if a > b false print a print b end Print the larger of 2 numbers 1BA3 – G Lacey – Semester 1 Lecture 4

6. Print the larger of two numbers • 10 input a • 20 input b • 30 if a >= b goto ? x • 40 print b • 50 goto ? y • x 60 print a • y 70 end 1BA3 – G Lacey – Semester 1 Lecture 4

7. Compute the sum of a series of positive numbers • Series is terminated by a negative number • Maximum number of integers in the series is 10 • This problem was solved in a previous lecture using SML 1BA3 – G Lacey – Semester 1 Lecture 4

8. sum = 0 counter = 0 read a true if a < 0 false true if counter == 10 print sum false counter +1 end sum + a Compute the sum of a series of positive numbers 1BA3 – G Lacey – Semester 1 Lecture 4

9. Compute the sum of a series of positive numbers 10 rem read series of numbers and print sum 12 rem initialise counter and sum 15 let c = 0 20 let s = 0 22 rem start programme 25 input x 30 if x > 0 goto ? 35 if c == 10 goto ? 40 let c = c + 1 45 let s = s + x 47 rem restart loop 50 goto 40 53 rem print sum and end programme 55 print s 60 end 1BA3 – G Lacey – Semester 1 Lecture 4

10. A SIMPLE to SML Compiler An introduction to the concepts in translating HLL into Assembly 1BA3 – G Lacey – Semester 1 Lecture 4

11. Compilers • Compilers translate High Level Languages (HLLs) like Java and C++ into Machine code • Two reasons for using HLLs • Ease of programming • It is easier to solve large problems using HLLs because most of the machine specific detail is hidden. • Machine Independence • Different processors use different instruction sets. This detail is taken care of by the compiler. Thus a c++ program written on a mac will also compile on a p.c. 1BA3 – G Lacey – Semester 1 Lecture 4

12. Translating Simple to Simpletron Machine Language • Compiling a Simple programme involves translating the Simple Code to SML • Each Simple statement has to be translated into SML instructions • Each variable must be allocated memory space • Each constant must be allocated memory space 1BA3 – G Lacey – Semester 1 Lecture 4

13. Compiling Simple • Allocating Data Space for variables and constants • start at the max memory and work down. • E.G. in Simpletron begin by allocating memory space 99, then 98, 97... • Compiling Instructions • Instructions are placed at the bottom of memory working up. • E.G. in Simpletron the processor always starts at 00. The first instruction is placed at 00, the next at 01, then 02 ... • Running out of space • If the programme is too big (too much code and too many variables and constants) the instruction space will run into the data space. 1BA3 – G Lacey – Semester 1 Lecture 4

14. Example Production Rules for translating Simple into SML • 10 input x • input corresponds to the READ operation SML thus the operator part of the SML instruction is 10 • x is a variable and must be allocated memory space if none has been allocated already. • the instruction placed in memory location 00 is 1099 • 20 print x • print corresponds to the WRITE operator in SML thus the operator is 11 • x has already been allocated memory location 99 • the instruction in memory location 01 is 1199 • 30 goto 10 • goto corresponds to the BRANCH operator in SML thus the operator is 40 • the instruction compiled from line number 10 was placed in memory location 00 thus the operand is 00 • the instruction 4000 is placed in memory location 02 • 40 end • corresponds to HALT in SML thus 4300 goes into location 03 1BA3 – G Lacey – Semester 1 Lecture 4

15. Example Compilation Simple Code SML Instructions 10 input x 20 print x 30 goto 10 40 end 1BA3 – G Lacey – Semester 1 Lecture 4

16. More Complex Production Rules • To translate if and goto statements the compiler must generate more than one statement. • First : • load x into the accumulator • Second : • branch if accumulator is zero to instruction from line number 10 Simple SML 10 input x 20 if x == 0 goto 10 1BA3 – G Lacey – Semester 1 Lecture 4

17. Another Example Simple SML 10 input x 20 input y 30 if x > y goto 10 1BA3 – G Lacey – Semester 1 Lecture 4