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Assembly Language Part 3. Symbols Symbols are assembler names for memory addresses Can be used to label data or instructions Syntax rules: –start with.

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Presentation on theme: "Assembly Language Part 3. Symbols Symbols are assembler names for memory addresses Can be used to label data or instructions Syntax rules: –start with."— Presentation transcript:

1 Assembly Language Part 3

2 Symbols Symbols are assembler names for memory addresses Can be used to label data or instructions Syntax rules: –start with letter –contain letter & digits –8 characters max –CASE sensitive Define by placing symbol label at start of line, followed by colon

3 Symbol Table Assembler stores labels & corresponding addresses in lookup table called symbol table Value of symbol corresponds to 1st byte of memory address (of data or instruction) Symbol table only stores label & address, not nature of what is stored Instruction can still be interpreted as data, & vice versa

4 Example Example program: this: deco this, d stop.end Output: What happened?

5 High Level Languages & Compilers Compilers translate high level language code into low level language; may be: –machine language –assembly language –for the latter an additional translation step is required to make the program executable

6 C++/Java example // C++ code: #include string greeting = “Hello world”; int main () { cout << greeting << endl; return 0; } // Java code: public class Hello { static String greeting = “Hello world”; public static void main (String [] args) { System.out.print (greeting); System.out.print(‘\n’); }

7 Assembly language (approximate) equivalent br main greeting:.ASCII "Hello world \x00" main: stro greeting, d charo '\n', i stop.end

8 Data types In a high level language, such as Java or C++, variables have the following characteristics: –Name –Value –Data Type At a lower level (assembly or machine language), a variable is just a memory location The compiler generates a symbol table to keep track of high level language variables

9 Symbol table entries The illustration drove shows a snippet of output from the Pep/8 assembler. Each symbol table entry includes: the symbol the value (of the symbol’s start address) the type (.ASCII in this case)

10 Pep/8 Branching instructions We have already seen the use of BR, the unconditional branch instruction Pep/8 also includes 8 conditional branch instructions; these are used to create assembly language control structures These instructions are described on the next couple of slides

11 Conditional branching instructions BRLE: –branch on less than or equal –how it works: if N or Z is 1, PC = operand BRLT: –branch on less than –how it works: if N is 1, PC = operand BREQ: –branch on equal –how it works: if Z is 1, PC = operand BRNE: –branch on not equal –how it works: if Z is 0, PC = operand

12 Conditional branching instructions BRGE: –branch on greater than or equal –if N is 0, PC = operand BRGT: –branch on greater than –if N and Z are 0, PC = operand BRV: –branch if overflow –if V is 1, PC = operand BRC: –branch if carry –if C is 1, PC = operand

13 Example Pep/8 code: br main num:.block 2 prompt:.ascii "Enter a number: \x00" main: stro prompt, d deci num, d lda num, d brge endif lda num, d nega; negate value in a sta num, d endif: deco num, d stop.end HLL code: int num; Scanner kb = new Scanner(); System.out.print (“Enter a number: ”); num = kb.nextInt(); if (num < 0) num = -num; System.out.print(num);

14 Analysis of example Pep/8 code: br main num:.block 2 prompt:.ascii "Enter a number: \x00" main: stro prompt, d deci num, d lda num, d brge endif lda num, d nega; negate value in a sta num, d endif: deco num, d stop.end The if statement, if translated back to Java, would now be more like: if (num >= 0); else num = -num; This part requires a little more explanation; see next slide

15 Analysis continued A compiler must be programmed to translate assignment statements; a reasonable translation of x = 3 might be: –load a value into the accumulator –evaluate the expression –store result to variable In the case above (and in the assembly language code on the previous page), evaluation of the expression isn’t necessary, since the initial value loaded into A is the only value involved (the second load is really the evaluation of the expression)

16 Compiler types and efficiency An optimizing compiler would perform the necessary source code analysis to recognize that the second load is extraneous –advantage: end product (executable code) is shorter & faster –disadvantage: takes longer to compile So, an optimizing compiler is good for producing the end product, or a product that will be executed many times (for testing); a non- optimizing compiler, because it does the translation quickly, is better for mid-development

17 Another example HLL code: final int limit = 100; int num; System.out.print(“Enter a #: ”); if (num >= limit) System.out.print(“high”); else System.out.print(“low”); Pep/8 code: br main limit:.equate 100 num:.block 2 high:.ascii "high\x00" low:.ascii "low\x00" prompt:.ascii "Enter a #: \x00" main: stro prompt, d deci num, d if: lda num, d cpa limit, i brlt else stro high, d br endif else: stro low, d endif: stop.end Compare instruction: cpr where r is a register (a or x): action same as subr except difference (result) isn’t stored in the register – just sets status bits – if N or Z is 0, <= is true

18 Writing loops in assembly language As we have seen, an if or if/else structure in assembly language involves a comparison and then a (possible) branch forward to another section of code A loop structure is actually more like its high level language equivalent; for a while loop, the algorithm is: –perform comparison; branch forward if condition isn’t met (loop ends) –otherwise, perform statements in loop body –perform unconditional branch back to comparison

19 Example The following example shows a C++ program (because this is easier to demonstrate in C++ than in Java) that performs the following algorithm: –prompt for input (of a string) –read one character –while (character != end character (‘*’)) write out character read next character

20 C++ code #include int main () { char ch; cout << “Enter a line of text, ending with *” << endl; cin.get (ch); while (ch != ‘*’) { cout << ch; cin.get(ch); } return 0; }

21 Pep/8 code ;am3ex5 br main ch:.block 1 prompt:.ascii "Enter a line of text ending with *\n\x00" main: stro prompt, d chari ch, d ; initial read lda 0x0000, i ; clear accumulator while: ldbytea ch, d ; load ch into A cpa '*', i breq endW charo ch, d chari ch, d ; read next letter br while endW: stop.end

22 Do/while loop Post-test loop: condition test occurs after iteration Premise of sample program: –cop is sitting at a speed trap –speeder drives by –within 2 seconds, cop starts following, going 5 meters/second faster –how far does the cop travel before catching up with the speeder?

23 C++ version of speedtrap #include int main() { int copDistance = 0; // cop is sitting still int speeder; // speeder's speed: entered by user int speederDistance;// distance speeder travels from cop’s position cout << "How fast is the driver going? (Enter whole #): "; cin >> speeder; speederDistance = speeder; do { copDistance += speeder + 5; speederDistance += speeder; } while (copDistance < speederDistance); cout << "Cop catches up to speeder in " << copDistance << " meters." << endl; return 0; }

24 Pep/8 version ;speedTrap br main cop:.block 2 drvspd:.block 2 drvpos:.block 2 prompt:.ascii "How fast is the driver going? (Enter whole #): \x00" outpt1:.ascii "Cop catches up to speeder in \x00" outpt2:.ascii " meters\n\x00" main: lda 0, i sta cop, d stro prompt, d deci drvspd, d; cin >> speeder; ldx drvspd, d; speederDistance = speeder; stx drvpos, d

25 Pep/8 version continued do: lda 5, i; copDistance += speeder + 5; adda drvspd, d adda cop, d sta cop, d addx drvspd, d; speederDistance += speeder; stx drvpos, d while: lda cop, d; while (copDistance < cpa drvpos, d; speederDistance); brlt do stro outpt1, d deco cop, d stro outpt2, d stop.end

26 For loops For loop is just a count-controlled while loop Next example illustrates nested for loops

27 C++ version #include int main() { int x,y; for (x=0; x < 4; x++) { for (y = x; y > 0; y--) cout << "* "; cout << endl; } return 0; }

28 Pep/8 version ;nestfor br main x:.word 0x0000 y:.word 0x0000 main:sta x, d stx y, d outer:adda 1, i cpa 5, i breq endo sta x, d ldx x, d inner:charo '*', i charo ' ', i subx 1, i cpx 0, i brne inner charo '\n', i br outer endo: stop.end

29 Notes on control structures It’s possible to create “control structures” in assembly language that don’t exist at a higher level Your text describes such a structure, illustrated and explained on the next slide

30 A control structure not found in nature Condition C1 is tested; if true, branch to middle of loop (S3) After S3 (however you happen to get there – via branch from C1 or sequentially, from S2) test C2 If C2 is true, branch to top of loop No way to do this in C++ or Java (at least, not without the dreaded goto statement)

31 High level language programs vs. assembly language programs If you’re talking about pure speed, a program in assembly language will almost always beat one that originated in a high level language Assembly and machine language programs produced by a compiler are almost always longer and slower So why use high level languages (besides the fact that assembly language is a pain in the patoot)

32 Why high level languages? Type checking: –data types sort of exist at low level, but the assembler doesn’t check your syntax to ensure you’re using them correctly –can attempt to DECO a string, for example Encourages structured programming

33 Structured programming Flow of control in program is limited to nestings of if/else, switch, while, do/while and for statements Overuse of branching instructions leads to spaghetti code

34 Unstructured branching Advantage: can lead to faster, smaller programs Disadvantage: Difficult to understand –and debug –and maintain –and modify Structured flow of control is newer idea than branching; a form of branching with gotos by another name lives on in the Java/C++ switch/case structure

35 Evolution of structured programming First widespread high level language was FORTRAN; it introduced a new conditional branch statement: if (expression) GOTO new location Considered improvement over assembly language – combined CPr and BR statements Still used opposite logic: if (expression not true) branch else // if-related statements here branch past else else: // else-related statements here destination for if branch

36 Block-structured languages ALGOL-60 (introduced in 1960 – hey, me too) featured first use of program blocks for selection/iteration structures Descendants of ALGOL include C, C++, and Java

37 Structured Programming Theorem Any algorithm containing GOTOs can be written using only nested ifs and while loops (proven back in 1966) In 1968, Edsgar Dijkstra wrote a famous letter to the editor of Communications of the ACM entitled “gotos considered harmful” – considered the structured programming manifesto It turns out that structured code is less expensive to develop, debug and maintain than unstructured code – even factoring in the cost of additional memory requirements and execution time


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