不管是大型電腦或個人電腦都需具有「中央處理單元」(central process unit,簡稱 CPU)。CPU 是電腦的「腦」,其電子電路負責處理所有軟體正確運作所需的所有任務,如算術、邏輯、控制、輸入和輸出操作等等。雖然早期的設計即可以讓一個指令同時做兩、三件不同的工作;但為了簡單化,我們在這裡所談的工作將只是執行算術和邏輯運算的工作(arithmetic and logic unit,簡稱 ALU),如將兩個數加在一起。在這一簡化的定義下,CPU 在任何一個時刻均只能執行一件工作而已。
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在個人電腦剛出現只能用於一般事物的處理時,CPU 均能非常勝任地完成任務。但電腦圖形和動畫的出現帶來了第一批運算密集型工作負載後,CPU 開始顯示心有餘而力不足:例如電玩動畫需要應用程式處理數以萬計的像素(pixel),每個像素都有自己的顏色、光強度、和運動等, 使得 CPU 根本沒辦法在短時間內完成這些工作。於是出現了主機板上之「顯示插卡」來支援補助 CPU。
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