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字符串常量（String constants）與某些僞指令一起使用，也就是類似DB或類似INCBIN等僞指令。又是也用在一些預處理指示中。
字符串常量與字符常量類似，只是可以更長（字符常量最多8個字節）。字符串常量被視爲在條件下允許最長的字符常量的連接。如下例兩條僞指令實際上定義的是相同的字符串常量
db 'hello' ; string constant
db 'h','e','l','l','o' ; equivalent character constants
下面例子也是一樣，三條僞指令都定義了三個doubleword長度的字符串常量。
dd 'ninechars' ; doubleword string constant
dd 'nine','char','s' ; becomes three doublewords
db 'ninechars',0,0,0 ; and really looks like this
注意，與僞指令一起使用時，即使字符串常量短到可以轉換成字符常量（長度小於8），也不會被當作字符常量。因爲如果不這樣db `ab`會等價於db `a`，產生這種現象是愚蠢的。類似的，座位DW操作數的長度是三和四的字符常量都會被當作字符串常量處理，以此類推。