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一個(gè)指針給出的是變量的地址，而不是變量的數(shù)值大小。在有些功能塊中，可能使用指針編程更為方便。用定義指針類型的形參，就能在功能塊中先使用一個(gè)虛設(shè)的指針，待調(diào)用功能塊時(shí)再為其賦予確定的地址。當(dāng)為指針參數(shù)類型形參分配實(shí)參時(shí)，需要指明內(nèi)存地址，例如P#M50.0。
3. 塊調(diào)用過(guò)程及內(nèi)存分配
CPU提供塊堆棧(B堆棧)來(lái)存儲(chǔ)與處理被中斷塊的有關(guān)信息。當(dāng)發(fā)生塊調(diào)用或有來(lái)自更高優(yōu)先級(jí)的中斷時(shí)，就有相關(guān)的塊信息存儲(chǔ)在B堆棧里，并影響部分內(nèi)存和寄存器。圖5.2顯示了調(diào)用塊時(shí)B堆棧與L堆棧的變化。圖5.3提供了關(guān)于STEP 7的塊調(diào)用情況。
1) B堆棧與L堆棧
B堆棧是CPU系統(tǒng)內(nèi)存中的一部分，它存儲(chǔ)以下被中斷塊的數(shù)據(jù)：
(1) 塊號(hào)、塊類型、優(yōu)先級(jí)、被中斷塊的返回地址；
(2) 塊寄存器DB、DI被中斷前的內(nèi)容；
(3) 臨時(shí)變量的指針(被中斷塊的L堆棧地址)。
STEP 7中可使用的B堆棧大小是有限制的，對(duì)于S7-300 CPU，則可在B堆棧中存儲(chǔ)8個(gè)塊的信息。因此，塊調(diào)用嵌套深度也是有限制的，多可同時(shí)激活8個(gè)塊。
L堆棧是CPU內(nèi)存中的一部分，它在塊調(diào)用時(shí)被重新分配。L堆棧用來(lái)存儲(chǔ)邏輯塊中定義的臨時(shí)變量，也分配給臨時(shí)本地?cái)?shù)據(jù)使用。梯形圖的方塊指令與標(biāo)準(zhǔn)功能塊也可能使用L堆棧存儲(chǔ)運(yùn)算的中間結(jié)果。
2) 調(diào)用功能塊FB
當(dāng)調(diào)用功能塊FB時(shí)，會(huì)有以下事件發(fā)生：
(1) 調(diào)用塊的地址和返回位置存儲(chǔ)在塊堆棧中，調(diào)用塊的臨時(shí)變量壓入L堆棧；
(2) 數(shù)據(jù)塊DB寄存器內(nèi)容與DI寄存器內(nèi)容交換；
(3) 新的數(shù)據(jù)塊地址裝入DI寄存器；
(4) 被調(diào)用塊的實(shí)參裝入DB和L堆棧上部；
(5) 當(dāng)功能塊FB結(jié)束時(shí)，先前塊的現(xiàn)場(chǎng)信息從塊堆棧中彈出，臨時(shí)變量彈出L堆棧；
(6) DB和DI寄存器內(nèi)容交換。
當(dāng)調(diào)用功能塊FB時(shí)，STEP 7并不一定要求給FB形參賦予實(shí)參，除非參數(shù)是復(fù)式數(shù)據(jù)類型的IO形參或參數(shù)類型形參。如果沒有給FB形參賦予實(shí)參，則功能塊FB就調(diào)用背景數(shù)據(jù)塊內(nèi)的數(shù)值。該數(shù)值是在功能塊的變量聲明表內(nèi)或背景數(shù)據(jù)塊內(nèi)設(shè)置的形參初始數(shù)值。
3) 調(diào)用功能塊FC
當(dāng)調(diào)用功能塊FC時(shí)會(huì)有以下事件發(fā)生：

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