COMMON A,400 ; int a[100] -- array of 100 words
PA: W A
; public information about fields of objects in class C
POP = 8 ; pointer to the parameterless pop() method
; it returns the popped value in R3
; activation record structure
I = 4 ; integer variable
PO = 8 ; pointer to the object O of class C
ARSIZE = 12
a) Write code to call o.pop(), leaving the result in R3 (1 point)
_LOAD__ _R3,R2,PO______ ; get pointer to o into a register
_LOAD__ _R1,R3,POP_____ ; get pointer to pop method
_ADDI__ _R2,R2,ARSIZE__ ; push the activation record
_JSRS__ _R1,R1_________ ; call the pop method
_ADDI__ _R2,R2,-ARSIZE_ ; pop the activation record
b) Write commented code to finish the assignment a[i] = o.pop() (1 point)Few did perfectly, the biggest problems were with register usage in the first two lines, frequently destroying R3 (an essential parameter to the method).
_LOAD__ _R4,PA_________ ; _Get pointer to A[]___
_LOAD__ _R5,R2,I_______ ; _Get local variable i__
_SR____ _R5,2__________ ; _Make i a byte offset__
_ADD___ _R4,R4,R5______ ; _Make pointer to A[i]__
_STORES _R3,R4_________ ; _A[i] = result from R3_
Only 1 did well here, and 3 left this blank. The common error was to attempt something with an array of bytes (A was explicitly described as an array of words).
| A | B | C D | |
|---|---|---|---|
| 0 | 0 | remember | |
| 0 | 1 | 1 0 | |
| 1 | 0 | 0 1 | |
| 1 | 1 | 0 0 | |
Where the inputs uniquely determine the outputs, enter values of 0 or 1. When the inputs allow the device to remember, write remember. (1 point)
b) Given your answer to part a), which of the flipflops that we discussed does this flipflop most closely mimic? (1/2 point)
_R_S______________________________
c) Show that you understand your answer to part b by marking, on the above figure, which inputs correspond to C, D, R, S (or whatever is appropriate) and which outputs correspond to Q or Q (whichever is appropriate). (1/2 point)
2 filled in the truth table correctly, 3 identified it as an RS flipflop, 4 identified it as some kind of R S flipflop, and the remainder gave odd answers. 4 labeled the inputs and outputs correctly.
|___|___|___|___|___|___|___|___|___|___|___|___| | s | exponent | mantissa |a) Give the decimal interpretation of the following values, expressed in hexadecimal: (1/2 point)
hex binary algebraic decimal
_3__
F80 _1 _1 _1 _1 _1 _0 _0 _0 _0 _0 _0 _0 __-1/2__ x 2 = _-4.0_____
_3__
7F0 _0 _1 _1 _1 _1 _1 _1 _1 _0 _0 _0 _0 __15/16_ x 2 = __7.5_____
_1__
580 _0 _1 _0 _1 _1 _0 _0 _0 _0 _0 _0 _0 ___1/2__ x 2 = __1.0_____
2 did well here, the most common error was to take the exponent as an unsigned integer, so that 111 was taken as 7 and not +3 (in the biased system). Some students invented strange and ornate variants on this system where they assumed hidden bits and other features of the IEEE system. 6 earned no credit at all.
b) Give the binary representations, in this system, for the following: (1/2 point)
The smallest positive normalized number: _0 _0 _0 _0 _1 _0 _0 _0 _0 _0 _0 _0
The smallest positive nonnormalized number: _0 _0 _0 _0 _0 _0 _0 _0 _0 _0 _0 _1
1 did well here. Strangely, many students tried to give the decimal values. Only 7 earned no credit at all.
a) Under what circumstances would a program (perhaps in main.a) explicitly mention BOXRENDER (1 point)
_When a call to b.render() is made for an object b that is_________ _known to be a box.________________________________________________
4 did well here, 8 earned no credit, and the remainder had vague answers of one kind or another.
b) a) Why, in a good solution to MP5, would it be unlikely to find explicit mention of BOXRENDER in main.a even though, when the main program is executed, it actually calls that subroutine many times? (1 point)
_The main program does not know the classes of the objects_________ _so it must call methods indirectl through the objects themselves._
4 did well here too. More students got partial credit here than in the first part.
3 ; fields common to box and text objects 4 METHODS = 0 ; pointer to method table of object's class 5 NEXT = 4 ; pointer to next object in list 6 7 ; structure of method table for box or text classes 8 ; each entry in the method table of a class is a pointer to a method 9 RENDER = 0 ; render the object 10 DEBUG = 0 ; debug the object
a) Under the old scheme, box.h included the text EXT BOXRENDER. What new symbols must be defined by EXT directives in the new header file for box.h? (1/2 point)
_BOXMETHODS,_BOXDEBUG______________________________________________
3 did well here, 9 mentioned just BOXDEBUG, several gave strange answers.
b) In the old MAKEBOX macro, there was a line W BOXRENDER; which of the new external symbols from part a will be used in the macro MAKEBOX? (1/2 point)
_BOXMETHODS________________________________________________________
5 did well here, many thought that there was some reason to include pointers to BOXRENDER or BOXDEBUG, indicating a misunderstanding of either the data structure or the role of the macro in the header file.
c) Given that R3 contains a pointer to an object of some concrete subclass of BOXORTEXT, possibly a BOX, write code to call the RENDER method of that object. Ignore the problem of passing parameters to the method other than the pointer to the object itself. (2 points)
__LOAD___R1,R3,METHODS____; get method table pointer of object_____ __ADDI___R2,R2,ARSIZE_____; push activation record_________________ __JSR____R1,R1,RENDER_____; call render method of object___________ __ADDI___R2,R2,-ARSIZE____; pop activation record__________________ __LOAD___R1,R3,METHODS____; get method table pointer of object_____ __LOAD___R1,R1,RENDER_____; get pointer to render method___________ __ADDI___R2,R2,ARSIZE_____; push activation record_________________ __JSRS___R1,R1____________; call the method________________________ __ADDI___R2,R2,-ARSIZE____; pop activation record__________________
3 did well here, most giving the second, longer solution. 8 more did well except that they forgot to push and pop the activation records. Those who had additional problems generally had difficulty with the data structures.