Assembly Language Part 5

Reference parameter/global variable model C++ reference parameters are references to the actual arguments (as opposed to copies of the values) At assembly language level, the address of the actual argument is pushed on the stack – in the previous example, the actual arguments were global variables; therefore we could use the name of the corresponding symbol to refer to the address – Code used to push the address of global variable is a load instruction with immediate addressing

A program with reference parameters #include int a, b; // global variables void swap (int& r, int& s) { int temp; temp = r; r = s; s = temp; } void order (int& x, int& y) { if (x > y) { swap (x, y); } // ra2 } int main () { cout > a; cout > b; order (a, b); cout << "Ordered they are: " << a << ", " << b << endl; // ra1 return 0; }

Pep/8 equivalent ;******* main () main: STRO msg1,d DECI a,d ;cin >> a STRO msg1,d DECI b,d ;cin >> b LDA a,i ;push address of a STA -2,s LDA b,i ;push address of b STA -4,s SUBSP 4,i ;push params CALL order ;order (a, b) ra1: ADDSP 4,i ;pop params STRO msg2,d DECO a,d STRO msg3,d DECO b,d CHARO '\n',i STOP msg1:.ASCII "Enter an integer: \x00" msg2:.ASCII "Ordered they are: \x00" msg3:.ASCII ", \x00".END int main () { cout > a; cout > b; order (a, b); cout << "Ordered they are: " << a << ", " << b << endl; // ra1 return 0; }

Pep/8 equivalent x:.EQUATE 4 ;formal parameter y:.EQUATE 2 ;formal parameter order: LDA x,sf ;if (x > y) CPA y,sf BRLE endIf LDA x,s ; push x STA -2,s LDA y,s ; push y STA -4,s SUBSP 4,i ; push params CALL swap ; swap (x, y) ADDSP 4,i ; pop params endIf: RET0 ;pop retAddr void order (int& x, int& y) { if (x > y) { swap (x, y); } // ra2 } This procedure (order) calls a second procedure, swap The order procedure received two reference parameters, x and y, whose addresses are already stored on the runtime stack Since the swap procedure is also expecting reference parameters, the order procedure simply passes on the address of x to swap

Pep/8 equivalent BR main a:.BLOCK 2 ;global variable b:.BLOCK 2 ;global variable ; ;******* void swap (int& r, int& s) r:.EQUATE 6 ;formal parameter s:.EQUATE 4 ;formal parameter temp:.EQUATE 0 ;local variable swap: SUBSP 2,i ;allocate local LDA r,sf ;temp = r STA temp,s LDA s,sf ;r = s STA r,sf LDA temp,s ;s = temp STA s,sf RET2 ;deallocate local, ;pop retAddr int a, b; // global variables void swap (int& r, int& s) { int temp; temp = r; r = s; s = temp; } Procedure swap needs to translate the line temp = r; into assembly language – meaning, it must access the value of a parameter whose address is on the runtime stack The mechanism for this access is stack-relative deferred addressing

Yet another addressing mode Recall that the relation between the operand and its specifier in stack-relative addressing is: operand = Memory[SP + operand specifier] – the key point is, the operand itself is stored on the runtime stack address – with reference parameters, the operand’s address is on the stack The addressing mode in use is stack-relative deferred addressing, in which the operand/specifier relation is: operand = Memory[Memory[SP + operand specifier]]

Argument is pushed on the stack via a load instruction with immediate mode addressing: LDA a, i STA -2, s Formal parameter is accessed using stack-relative deferred addressing: LDA r, sf ; temp = r; STA temp, s ; reference parameter on right side of = ; with local variable on left side LDA s, sf ; r = s; STA r, sf ; reference parameters on both sides of = Translating pass-by-reference calls involving global variables: summary

More common situation: reference parameter with local variables // C++ program with method for finding perimeter of rectangle #include void rect (int& p, int w, int h) { p = (w + h) * 2; } int main () { int perim, width, height; cout > width; cout > height; rect (perim, width, height); // ra1 cout << "perim = " << perim << endl; return 0; }

Pep/8 equivalent Procedure rect uses its reference parameter, p, as we saw with the previous example: STA p, sf stores the value in A using stack-relative deferred addressing Note the line before that: what does ASLA do? ;File: fig0629.pep ;Computer Systems ;Figure 6.29 ; BR main ; ;******* void rect (int& p, int w, int h) p:.EQUATE 6 ;formal parameter w:.EQUATE 4 ;formal parameter h:.EQUATE 2 ;formal parameter rect: LDA w,s ;p = (w + h) * 2 ADDA h,s ASLA STA p,sf endIf: RET0 ;pop retAddr ;

Pep/8 equivalent: first part of main Local variables perim, width and height are pushed on runtime stack The symbol perim is not the absolute address of its value, as it was in the global case; its value (4) is only the address relative to the stack top – so can’t use: LDA perim, i STA -2, s The instructions: MOVSPA ADDA perim, i STA -2, S are used to : – copy SP value to A – Add value of perim (4) to A – Put address of perim in stack cell that will be the reference parameter, p ;******* main () perim:.EQUATE 4;local variable width:.EQUATE 2;local variable height:.EQUATE 0;local variable main: SUBSP 6,i;allocate locals STRO msg1,d DECI width,s STRO msg2,d DECI height,s MOVSPA ;push address of perim ADDA perim,i STA -2,s LDA width,s ;push value of width STA -4,s LDA height,s ;push value of height STA -6,s SUBSP 6,i ;push params

Pep/8 equivalent: rest of main CALL rect ;rect (perim, width, height) ra1: ADDSP 6,i ;pop params STRO msg3,d DECO perim,s CHARO '\n',i ADDSP 6,i ;deallocate locals STOP msg1:.ASCII "Enter width: \x00" msg2:.ASCII "Enter height: \x00" msg3:.ASCII "perim = \x00".END

Value-returning method The previous example was typical in the sense that most programs use local variables rather than global, but atypical in its use of a reference parameter in a void method to change one value It is usual in this sort of situation (one value to be changed) to write a function – that is, a value- returning method or procedure Pages of your text illustrate a value- returning method with an added twist: recursion

Translating boolean values C++ treats boolean values as 0 (meaning false) and anything else (meaning true) Java treats these values similarly, if not transparently At the bit level, it would seem obvious to extend this practice: – 0000 means false – 0001 means true

The complications of convenience Consider the basic logical operations AND, OR and NOT – 0000 AND 0001 is 0000 (false, as it should be) – 0000 OR 0001 is 0001 (true, also as expected) – NOT 0000 is 1111 – oops… Many architectures resolve this by using XOR instead of NOT, since – p 0 = p and – p 1 = !p

Indexed addressing and arrays Recall these basic concepts concerning variables: – A variable in HLL is a memory chunk at machine level – In HLL, we refer to variable by name; at machine level, we refer to it by address – A variable in assembly language may be referred to by name, but the value of that name is the address of the memory chunk

Indexed addressing and arrays An array at either level is just a bigger chunk of memory At the high level, we refer to an array by name, and its individual elements by index – The index is an offset added to the initial address (referred to by name) – The amount added is the index multiplied by the size of the data item At the machine architecture and assembly levels, we refer to the address of the array’s start – in assembly language, the symbol of an array is the address of the first element

Arrays and addressing modes Pep/8 programs with arrays use the following addressing modes: – indexed: Memory[Op specifier + X] – stack-indexed: Memory[SP + Op specifier + X] – stack-indexed deferred: Memory [Memory[SP + Op specifier] + X] Each element of an array is accessed by: – loading its index into X – multiplying by the number of bytes per cell – using stack-indexed addressing

Local array variables & array parameters Local array variables are allocated on the runtime stack during program execution: – SUBSP allocates space for array – ADDSP deallocates it To pass array as parameter: – address of first element is pushed on runtime using MOVSPA and ADDA with immediate addressing – Element is accessed by loading its index into X, multiplying by # bytes per cell, and using stack- indexed deferred addressing