雙語暢銷書《艾倫圖靈傳》第8章:水銀延時線(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.
這一點乍看起來有點不可思議，那就是ACE不具備條件分支能力，In this respect it might seem to lack the crucial idea that Babbage had introduced a hundred years earlier.
而這正是巴貝奇在100年前提出來的最重要的思想。
For the 'scanner', or Logical Control, of the ACE could only hold one 'address', or position on the tape, at a time.
因爲ACE的掃描器只能定位一個"地址"，也就是當前在紙帶上的位置，It had no way of holding more than one, and then of selecting a next destination according to some criterion.
它不能定位多個地址，所以也就不能根據某些條件來決定下一個目標。The omission, however, was only on the surface.
然而，這其實只是一個表面上的區別。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.
圖靈開拓了一條新的道路，他通過一種不需要定位多個地址的方式，巧妙地實現了條件分支的功能，It was not the best technical solution, but it had the ment of a brutal simplicity.
這樣做的代價是需要更多的指令存儲空間，但好處是簡化了硬件設備。Suppose that it were wished to execute instruction 50 if some digit D were 1, and to execute instruction 33 if it were 0.
舉例來說，假如分支的判定條件是數字D，如果D等於1，則執行指令50，如果D等於0，則執行指令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.
那麼圖靈的想法則是，把指令本身看成一個數，那麼D× （指令50）+（1 - D）× （指令33）的計算結果，正是需要執行的指令。The 'IF' was effected not by hardware, but by extra programming.
圖靈通過增加程序，而不是硬件，來實現"如果"的功能。It was a device which had led him to mix up data (the digit D) with instructions.
他只增加了一個裝置，以便實現數據（比如上例中的數字D）與指令的運算。This in itself was of great significance, for he had allowed himself to modify the stored program.
這個想法本身，有着非凡的意義，這使得在機器中運行的程序，可以根據需要發生改變。But this was only the beginning.
不過，這還只是開始。
