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Date: Tue, 05 Nov 1996 00:32:21 GMTServer: NCSA/1.5Content-type: text/htmlLast-modified: Fri, 16 Aug 1996 15:58:20 GMTContent-length: 13462<html><head><title> Lecture notes - Chapter 2 - SAL</title></head><BODY><h1> Chapter 2 -- SAL</h1><pre>ABOUT SAL---------MOTIVATION for SAL: hiding the details of MAL, MIPS asm. lang. (on purpose!) SAL code will look more like HLL code -- to make student's transition easier. Introducing one more level of abstraction in order to postpone discussion of several topics. HLL SAL assembly machine code each HLL statement maps into 1 or MORE SAL instructions each SAL instruction maps into 1 or MORE MAL instructions for this course, there is actually one more layer that students will be given HLL SAL MAL TAL MIPS RISC machine codeSALA subset of the functionality of most high level languages -- no records/structures no formal arrays (see chapter 7 for their implementation)What is required by a programming language? declarations arithmetic operations conditional execution (if then else) looping control structures communication w/user. . .(write statement)About SAL: -- one instruction, declaration per line -- comments are anything on a line following `#' (comments may not span lines) DECLARATIONS------------ - they give information about how much memory space is needed - they assign a name to the memory space SAL has 3 basic types: integer, float (real), character can build other types out of these, for example, boolean is really an integer with only 2 defined values. Pascal: var variablename: type; C: type variablename; SAL: variablename: type value type is .word if integer .byte if character .float if real (floating point) value is optional -- it gives the variable an initial value examples: flag: .word 0 counter: .word 0 variable3: .word e: .float 2.71828 uservalue: .byte letter: .byte 'a'other useful rules: -- one declaration per line. -- default initial value is 0. DIRECTIVES---------- a way to give information to the assembler. - all directives start with `.' (period) examples: .byte .word .float .data # identifies the start of the declaration section # there can be more than 1 .data section in # a program .text # identifies where instructions are # there can be more than 1 .text section in # a program .asciiz "a string.\n" # places a string into memory # and null terminates the string. .ascii "new string." # places a string into memory # WITHOUT null termination. ARITHMETIC operations---------------------- SAL Pascal C move x, y x := y; x = y; add x, y, z x := y + z; x = y + z; sub x, y, z x := y - z; x = y - z; mul x, y, z x := y * z; x = y * z; div x, y, z x := y div z; x = y / z; rem x, y, z x := y mod z; x = y % z; NOTES: 1. the operation result depends on the type of the variables. 2. cannot increase the number of operands. 3. y and/or z can be IMMEDIATES examples: move count, 0 mult product, mult1, mult2 add sum, 2, addendNOTE: there are other instructions that implement boolean functions, but we don't cover them yet. CONDITIONAL EXECUTION--------------------- sometimes an instruction (or a set of instructions) should be executed, and sometimes it (they) shouldn't. HLL -- simplest form is a go-to. (Always discouraged.)Pascal if-then-else (a conditional go-to!) if (condition) then statement else statement;C if-then-else if (condition) statement; else statement;SAL 'ifs' and 'gotos'-------------------------------- SAL Pascal b label goto label; bltz x, label if x < 0 then goto label; bgtz x, label if x > 0 then goto label; blez x, label if x <= 0 then goto label; bgez x, label if x >= 0 then goto label; beqz x, label if x = 0 then goto label; bnez x, label if x <> 0 then goto label; beq x, y, label if x = y then goto label; bne x, y, label if x <> y then goto label; blt x, y, label if x < y then goto label; bgt x, y, label if x > y then goto label; ble x, y, label if x <= y then goto label; bge x, y, label if x >= y then goto label; EXAMPLE:-------- Pascal if-then-else: if (count < 0) then begin count := count + 1; end; C equivalent: if (count < 0) count = count + 1;SAL equiv to if-then-else: bltz count, ifstuff b endif ifstuff: add count, count, 1 endif: # next program instruction goes here -- OR -- bgez count, endif add count, count, 1 endif: # next program instruction goes here WHICH ONE OF THESE IS BETTER? Structured loops can be built out of IF's and GOTO's (test and branch)EXAMPLES:---------while loop example Pascal: while ( count > 0 ) do begin a := a mod count; count := count - 1; end; BAD STYLE Pascal: while: if (count <= 0) then goto endwhile; a := a mod count; count := count - 1; goto while; endwhile: C: while (count > 0) { a = a % count; count --; } SAL: while: blez count, endwhile rem a, a, count sub count, count, 1 b while endwhile: # next program instruction goes here repeat loop example Pascal: /* do statement until expression is TRUE */ repeat if (a < b) then a := a + 1; if (a > b) then a := a - 1; until a = b; C: /* do statement while expression is TRUE */ /* when expression is FALSE, exit loop */ do { if (a < b) a++; if (a > b) a--; } while( a != b); SAL: repeat: bge a, b, secondif add a, a, 1 secondif: ble a, b, until sub a, a, 1 until: bne repeat while loop example Pascal: while (count < limit) and (c = d) do begin /* loop's code goes here */ end; C: while ( (count<limit) && (c==d) ) { /* loop's code goes here */ } SAL: while: bge count, limit, endwhile bne c, d, endwhile # loop's code goes here b while endwhile:for loop example Pascal: for i:= 3 to 8 do begin a := a + i; end; C: for ( i = 3; i <= 8; i++) { a = a + i; } SAL: move i, 3 for: bgt i, 8, endfor add a, a, i add i, i, 1 b for endfor: COMMUNICATION WITH THE USER (I/O operations)-------------------------------------------- SAL Pascal put x write(x); get x readln(x); puts 'string' write('string');SAL doesn't have any oddities abouttesting for eoln or eof. A carriage return is just anothercharacter to be read or written. It is '\n'get has the most irregularities!--- get intvariable places first integer variable it "finds" into intvariable, then throws away the rest of the line. if the first value read does not match the type requested, then the value 0 is placed in variable intvariable. get charvariable placed the first character variable it has into charvariableEXAMPLES: INPUT: 23 abc -13 1234fgh! SAL CODE: get int1 # int1 <- 23 get int2 # int2 <- -13 get int3 # int3 <- 1234 OR: get char1 # char1 <- '2' get int1 # int1 <- 3, and throw away rest of line get char2 # char2 <- ' ', there is a space character before # the -13 OR: get char1 # char1 <- '2' get char2 # char2 <- '3' get int1 # int1 <- 0, because the first value read (ignoring # the white space) is not a digit, so the type does # not match.So, if you want to get more than one non-character value froma single line, you must read in character by character, andconvert to whatever form is desired. More about this inchapter 4. PROGRAM EXAMPLE:----------------# this simple program adds up 2 integers and prints their sum and products. .data prompt1: .asciiz "Enter an integer: " prompt2: .asciiz "Enter a second integer: " linefeed: .byte '\n' msg1: .asciiz "The sum of " msg2: .asciiz " and " msg3: .asciiz " is " msg4: .asciiz "The product of " int1: .word 0 int2: .word 0 sum: .word product: .word .text # get the 2 integers from user puts prompt1 get int1 put linefeed puts prompt2 get int2 put linefeed # calculate the sum and products add sum, int1, int2 mul product, int1, int2 # print out the sum and products puts msg1 put int1 puts msg2 put int2 puts msg3 put sum put linefeed puts msg4 put int1 puts msg2 put int2 puts msg3 put product put linefeed done Another simple SAL example program----------------------------------# A SAL program to print out a multiplication table .datastart: .word 0 # entered by userfinish: .word 0 # entered by userii: .word # loop induction variablejj: .word # loop induction variableproduct: .wordprompt1: .asciiz "Enter starting value: "prompt2: .asciiz "Enter ending value: "newline: .byte '\n'x_symbol: .byte 'X'equals: .byte '='space: .byte ' ' .text__start: puts prompt1 # get user input get start puts prompt2 get finish move ii, startfor: bgt ii, finish, all_done # nested for loop to print out move jj, start # the tablenested: bgt jj, finish, next_iter mul product, ii, jj # print one line of table put ii put space put x_symbol put space put jj put space put equals put space put product put newline add jj, jj, 1 b nestednext_iter: add ii, ii, 1 put newline b forall_done: done PROCEDURES----------SAL has a rudimentary method for procedure call and return.There is no mechanism for parameter passing or function returnvalues.procedure parts: 1. call SAL procedure call: b procname 2. execute procedure procname: #code for procedure goes here 3. return "the hard part" could be a branch, if we had a label for the return place. b procname rtn1: # more code here procname: # procedure code here . . . b rtn1 This does not work. It just jumps to a different place in the code! It cannot be called from more than one location. Needed: an ADDRESS to return to!Convenient SAL instruction: la var1, label la stands for Load Address Operation: the address implied by label is placed into var1. NOTE: var1 must be declared as an integer (.word) Notice difference between address and contents of the address. label address contents aa: 103 6 bb: 104 'a' cc: 105 2001 The SAL instruction la cc, bb changes the table above to be: label address contents aa: 103 6 bb: 104 'a' cc: 105 ** 104 **For procedure call and return, save a return addressbefore branching to the procedure. la procname_ret, rtn1 b procname rtn1: # more code here . . . procname: # procedure code here . . . b procname_ret THIS STILL DOESN'T WORK! It branches to label procname_ret. But, procname_ret is a variable! Can't branch to a variable! To solve the problem, there is a special form of the b instruction used just for procedure return. b (var1) #parentheses identify the special form branches to the contents of var1, not to var1 itself.So, the complete call/return code is: la procname_ret, rtn1 b procname rtn1: # more code here . . . la procname_ret, rtn2 b procname rtn2: # more code here . . . procname: # procedure code here . . . b (procname_ret)</pre>⌨️ 快捷键说明
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