如何使用jr指令创建跳转表? [英] How to create a jump table using a jr instruction?

问题描述

C ++程序

 # include < iostream >
 # include <string >

 using namespace std;

 int main ()
 {
 int resistance ; // in Ohms
 string partNum ; // Part Number

 cout << " Enter resistance : " << endl ;
 cin >> resistance ;
 switch ( resistance )
 {
 case 3 : partNum = " OD30GJE "; break ;
 case 10 : partNum = " OD100JE "; break ;
 case 22 : partNum = " OD220JE "; break ;
 default : partNum = "No match "; break ;
 }    
 cout << " Part number : " << partNum << endl ;    
 return 0;
 }

将C代码转换为MIPS汇编代码，并向您的代码添加与最接近的电阻器匹配的功能.确保将jr指令用于switch语句.让您的代码从用户那里获得电阻作为输入，并在控制台上显示电阻的相应或最接近的零件号.

Translate the C code to MIPS assembly code, add to your code the capability to match the closest resistor. Be sure to use the jr instruction for the switch statement. Have your code get the resistance as input from the user and display to the console the resistor's corresponding or closest part number.

Mips汇编代码

.data
int_value: .space 20
.align 2
input:  .asciiz "Enter resistance.\n"       # declaration for string variable, 
string1:    .asciiz "OD30GJE\n" # declaration for string variable, 
string2:    .asciiz "OD100JE\n"
string3:    .asciiz "OD220JE\n"
string11:   .asciiz "No Match\n"
string12:   .asciiz "Enter resistance\n"
    .text
main:
li $v0, 4
la $a0, input                   # print for input 
syscall

la      $t0, int_value
li  $v0, 5          # load appropriate system call code into register $v0;

syscall             # call operating system to perform operation
sw  $v0, int_value      # value read from keyboard returned in register $v0;
                        # store this in desired location
lw  $s1, 0($t0)
condition1:
slt $t1, $s1, $zero # if $s1 < 0 $t1 = 1 else $t1 = 0
beq $t1, $zero, condition2 # if $t1 = 0; InvalidEntry 
bne $t1, $zero, invalid_entry

condition2:
sgt $t1, $s1, -1 # if $s1 > -1 then $t1 = 1 else $t1 = 0
beq  $t1, $zero, invalid_entry # if $t1 = 0; InvalidEntry 
sgt $t1, $s1, 9 # if s1 > 9 t1 = 1 else $t1 = 0 
bne $t1, $zero, condition3 # if $t1 does not equal = 0; condition3 

li  $v0, 4              
la  $a0, string1
syscall
j exit

condition3:
sgt $t1, $s1, 9 # if $s1 > 9 then $t1 = 1 else $t1 = 0
beq  $t1, $zero, invalid_entry # if $t1 = 0; InvalidEntry 
sgt $t1, $s1, 21 # if s1 > 21 t1 = 1 else $t1 = 0 
bne $t1, $zero, condition3 # if $t1 does not equal = 0; condition3 

li  $v0, 4              
la  $a0, string2
syscall
j exit

invalid_entry:
li  $v0, 4              
la  $a0, string11
syscall
j exit
exit:
li $v0, 10 # v0<- (exit)
syscall

推荐答案

下面显示了如何创建跳转表.假定跳转表中的偏移量在$s1. $s1中，必须为4的倍数(即字节偏移量).

The following shows how to make a jump table. The offset within the jump table is assumed to be in $s1. $s1 must be a multiple of 4 (i.e. a byte offset).

此代码未经测试！

        . . . .
        b       1f            # jump past the jump table
        nop                   # branch delay slot
        # table of jump addresses
JTAB:   .word LABEL1
        .word LABEL2
        .word LABEL3
1:      la      $t0, JTAB     # load start address of the jump table
        add     $t0, $t0, $s1 # add offset to table address
        lw      $t1, 0($t0)   # load the address stored at JTAB + $s1
        jr      $t1           # and jump to that address
        nop                   # branch delay slot
LABEL1: # do something
        b       2f            # break
        nop
LABEL2: # do something else
        b       2f            # break
        nop
LABEL3: # do a different thing
        b       2f            # break
        nop
2:      # after the end of the "case statement"

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