双语畅销书《艾伦图灵传》第8章:水银延时线(20)
Although the ACE was based upon the idea of the Universal Turing Machine, in one way it departed from it.
The departure lay in the feature, at first sight an extraordinary one, that it had no facility for conditional branching.
这一点乍看起来有点不可思议，那就是ACE不具备条件分支能力，In this respect it might seem to lack the crucial idea that Babbage had introduced a hundred years earlier.
而这正是巴贝奇在100年前提出来的最重要的思想。
For the 'scanner', or Logical Control, of the ACE could only hold one 'address', or position on the tape, at a time.
因为ACE的扫描器只能定位一个"地址"，也就是当前在纸带上的位置，It had no way of holding more than one, and then of selecting a next destination according to some criterion.
它不能定位多个地址，所以也就不能根据某些条件来决定下一个目标。The omission, however, was only on the surface.
然而，这其实只是一个表面上的区别。Alan set out a way in which conditional branching could be done without needing the logical control to hold more than one 'address' at a time.
图灵开拓了一条新的道路，他通过一种不需要定位多个地址的方式，巧妙地实现了条件分支的功能，It was not the best technical solution, but it had the ment of a brutal simplicity.
这样做的代价是需要更多的指令存储空间，但好处是简化了硬件设备。Suppose that it were wished to execute instruction 50 if some digit D were 1, and to execute instruction 33 if it were 0.
举例来说，假如分支的判定条件是数字D，如果D等于1，则执行指令50，如果D等于0，则执行指令33。His idea was to 'pretend that the instructions were really numbers and calculate D × (Instruction 50) + (1-D) × (Instruction 33).' The result of this calculation would be a instruction which would have the effect that was required.
那么图灵的想法则是，把指令本身看成一个数，那么D× （指令50）+（1 - D）× （指令33）的计算结果，正是需要执行的指令。The 'IF' was effected not by hardware, but by extra programming.
图灵通过增加程序，而不是硬件，来实现"如果"的功能。It was a device which had led him to mix up data (the digit D) with instructions.
他只增加了一个装置，以便实现数据（比如上例中的数字D）与指令的运算。This in itself was of great significance, for he had allowed himself to modify the stored program.
这个想法本身，有着非凡的意义，这使得在机器中运行的程序，可以根据需要发生改变。But this was only the beginning.
不过，这还只是开始。
