ICS Fall 2023

Appearance

Return to top
On this page

作业五

作业说明

  • 作业可以使用中文或英文完成，请勿抄袭
  • BB 系统上请提交 电子版 PDF，允许 LaTeX、Markdown、Word、手写拍照等
  • Deadline: 12 月 03 日 23:30，逾期 0 分
  • 本次作业 PDF
  • 答案 已发布

T1

What is the purpose of the .END pseudo-op? How does it differ from the HALT instruction?

T2

What are the definitions of a queue?

T3

The following program has an error in it. What is the error? How would you fix it?

    .ORIG x3000
A   .FILL xDEAD
B   .FILL xBEEF
    LD R0, A
    ST R0, B
    HALT
    .END

T4

Suppose you write two separate assembly language modules that you expect to be combined by the linker. Each module uses the label AGAIN, and neither module contains the pseudo-op .EXTERNAL AGAIN. Is there a problem using the label AGAIN in both modules? Why or why not?

T5

Your friend has just written a simple program intended to calculate complements, which is as follows:

    .ORIG x3000
    ; Simple program that should calculate 
    ; complement of DATA and store the result back
    LD R2, DATA
    NOT R2, R2
    ADD R2, R2, #1
    ST R2, DATA
DATA .FILL xF001
    .END

However, it does not seem to be reliable for some reason...

Questions:

  1. What's the 2's complement of xF001 in hex?
  2. Will the program store the complement to DATA?
  3. What will happen afterwards? Why?

Open questions (Answer if you like, but it WILL NOT be graded): What's the root cause of this phenomenon? How can we prevent this from happening?

T6

What's the difference between pseudo-ops .FILL, .BLKW and .STRINGZ in LC3?

T7

It is often useful to find the midpoint between two values. For this problem, assume A and B are both even numbers, and A is less than B. For example, if A = 2 and B = 8, the midpoint is 5. The following program finds the midpoint of two even numbers A and B by continually incrementing the smaller number and decrementing the larger number. You can assume that A and B have been loaded with values before this program starts execution.

Your job: Insert the missing instructions.

    .ORIG x3000
    LD R0, A
    LD R1, B
X   ________________ (a)
    ________________ (b)
    ADD R2, R2, R1
    ________________ (c)
    ADD R1, R1, #-1
    ________________ (d)
    BRnzp X
DONE ST R1,C
    TRAP x25
A   .BLKW 1
B   .BLKW 1
C   .BLKW 1
    .END

T8

We all know that we can achieve left-shift by adding the number to itself. For example, ADD R0, R0, R0 will left-shift R0 by 1 bit. However, right-shift is not that easy. Complete the following LC3 program so that it will right-shift R0 by 1 bit. Note that some comments have been deleted.

    .ORIG x3000
    ; Suppose R0 is already loaded with the target number
    ; Initialize
    AND R1, R1, #0      ; Result
    ADD R2, R1, #15     ; Loop var i
    ADD R3, R1, #__ (a) ; 1 << (**DELETED**)
    ADD R4, R1, #1      ; 1 << (15 - i)
    AND R5, R5, #0      ; Temp result
    ; Main Loop
L   AND R5, R3, R0      ; Test bit
    BR___ (b) N         ; **DELETED**
    ADD R1, R1, R4      ; Add to result
N   ADD R3, __, __ (c)  ; **DELETED**
    ADD R4, R4, R4      ; L-shift R4
    ADD __, __, __ (d)  ; **DELETED**
    BRp L
    ; End
    HALT
    .END

T9

The following operations are performed on a stack:

PUSH A
PUSH B
POP
PUSH C
POP
PUSH D
PUSH E
PUSH F
POP
PUSH G
POP
POP
POP
PUSH H
  1. What dose the stack contain after the PUSH H ?
  2. At which point does the stack contain the most element?

Without removing the element left in the stack from the previous operations, we change this stack to a queue (the front of queue is the top of stack), and perform

ENQUEUE I
DEQUEUE
ENQUEUE J
ENQUEUE K
DEQUEUE
ENQUEUE L
DEQUEUE
DEQUEUE
DEQUEUE
DEQUEUE
ENQUEUE M
DEQUEUE
  1. What does the stack contain now?

T10

Write a function that implements another stack function, PEEK. PEEK returns the value of the top element of the stack without removing the element from the stack. The return value is stored in R0, so you don't need to save R0. PEEK should also do underflow error checking: if an underflow occurs, you should output the string "Stack underflow error" and halt. (Suppose the pointer of top of the stack is in R6, and the stack can only take up the memory space from x3FFF to x3FF0)