IEEE.754在手臂上，组装算法给出意外结果

Question

。预期的结果是R0寄存器在击中

BX lr

时为

0x40000000，这是该程序的终结。我现在得到的结果是令人失望的0x40800000.。

I编译：

-arm-non-eabi-as -g -o float.ofloat.s

Arm-None-EABI-LID -0 floatfloat.o

然后我和Qemu

一起跑步

Qemu -arm -G 30005./part5

与GDB

一起走

GDB-Multiarch文件./float Target遥控器：30005

与GDB

一样正常

.section .data

@ Constants
exponent_mask: .word 0x7F800000

@ The two numbers we want to add
num1:   .word   0x3f800000
num2:   .word   0x3f800000 

.section .text
.global main

main:
@ Load numbers directly
    LDR r0, =num1
    LDR r0, [r0]
    LDR r1, =num2
    LDR r1, [r1]



@ Load constant for exponent extraction just once
    LDR r12, =exponent_mask
    LDR r12, [r12]

@ Extract exponents
    AND r2, r0, r12
    AND r3, r1, r12
    LSR r2, r2, #23
    LSR r3, r3, #23

@ Clear the exponent parts
    BIC r4, r0, r12
    BIC r5, r1, r12

@ Add the implicit 1 to the fractions
    ORR r4, r4, #0x00800000  
    ORR r5, r5, #0x00800000  


@ Align exponents and adjust fractions
    CMP r2, r3
    BGT largerExponent

@ r3 has larger exponent
    SUB r6, r3, r2
    LSR r4, r4, r6
    MOV r2, r3
    B fractionsAligned

largerExponent:
    SUB r6, r2, r3
    LSR r5, r5, r6

fractionsAligned:
    ADD r4, r4, r5   @ Add aligned fractions

@ Normalize if needed
    TST r4, #0x01000000
    BEQ doneNormalize
    LSR r4, r4, #1
    ADD r2, r2, #1

doneNormalize:
    @ Construct the final number
    LSL r2, r2, #23
    ORR r0, r4, r2

BX lr

在程序的末尾，寄存器值看起来像这样：

REG值

添加和归一化后，您的代码不会删除领先的1位。