（1）编写一个伪代码算法，读入三个值，输出它们的和：
Set sum to0
Read num1
Set sum to sum + num1
Read num2
Set sum to sum + num2
Read num3
Set sum to sum + num3
If (sum<0)
Write"Error"
Else
Write sum
（2）用PEP/9 机器指令实现（1）中的算法，提交测试结果截图：
12 00 0B 00 00 00 00 00 00 00 00 31 00 09 31 00
07 C1 00 03 61 00 07 E1 00 03 C1 00 05 60 00 01
E1 00 05 A1 00 09 18 00 2C 12 00 0E 39 00 03 00
zz
（3）用PEP/9 汇编语言实现（1）中的算法，提交测试结果截图：
BR main ; Branch to main program
sum: .WORD 0x0000 ; Set up sum and initialize to zero
counter: .WORD 0x0000 ; Set up the counter
num: .BLOCK 2 ; Set up a twe byte block for num
limit: .BLOCK 2 ; Set up a twe byte block for limit
main: DECI limit,d ; Read the number of values to add up
loop: DECI num,d ; Read a number
LDWA sum,d ; Load the current sum into the accumulator
ADDA num,d ; Add the number to the sum
STWA sum,d ; Store the sum
LDWA counter,d ; Load the counter into the accumulator
ADDA 1,i ; Add one to the counter
STWA counter,d ; Store the counter
CPWA limit,d ; Compare counter and limit
BREQ finish ; Branch to finish if conter has reached limit
BR loop; ; Otherwise, branch to loop to read another number
finish: DECO sum,d ; Output sum
STOP ; Stop
.END
