COBOL经典面试题库(中英文版).doc

COBOL经典面试题库（中英文版）Q1) Name the divisions in a COBOL program ?.A1) IDENTIFICATION DIVISION, ENVIRONMENT DIVISION, DATA DIVISION, PROCEDURE DIVISION.Q：列举COBOL的DEVISIONA：标识部，环境部，数据部，过程部Q2) What are the different data types available in COBOL?A2) Alpha-numeric (X), alphabetic (A) and numeric (9).Q：COBOL有哪些可用的数据类型A：字符型（这里指的是包含字母和数字），字母型，数字型Q3) What does the INITIALIZE verb do? GSA3) Alphabetic, Alphanumeric fields & alphanumeric edited items are set to SPACES. Numeric, Numeric edited items set to ZERO. FILLER , OCCURS DEPENDING ON items left untouched.Q：INITIALIZE这个词做了些什么A：将字母，字符，数字区域都置成空格（置空），将数字区置0， FILLER和OCCURS DEPENDING ON项不处理Q4) What is 77 level used for ?A4) Elementary level item. Cannot be subdivisions of other items (cannot be qualified), nor can they be subdivided themselves.Q：77层有什么作用A：基本层数据项，不能用做细分别的层，也不能被细分（来源：）Q5) What is 88 level used for ?A5) For condition names.Q：88层有什么作用A：条件逻辑层Q6) What is level 66 used for ?A6) For RENAMES clause.Q：66层有什么作用A：重命名层Q7) What does the IS NUMERIC clause establish ?A7) IS NUMERIC can be used on alphanumeric items, signed numeric & packed decimal items and unsigned numeric & packed decimal items. IS NUMERIC returns TRUE if the item only consists of 0-9. However, if the item being tested is a signed item, then it may contain 0-9, + and .Q：IS NUMERIC这个子句怎么确定（也就是说确定句子的真值）A：IS NUMERIC用在字符项，带符号数字，浮点数，不带符号数。如果目标项只含09则返回TRUE。但是，如果待测项目是个带符号数，那么他就含有0-9还有+和-Q8) How do you define a table/array in COBOL?A8) ARRAYS.05 ARRAY1 PIC X(9) OCCURS 10 TIMES.05 ARRAY2 PIC X(6) OCCURS 20 TIMES INDEXED BY WS-INDEXQ：COBOL中怎么建表/数组A：如上.Q9) Can the OCCURS clause be at the 01 level?A9) No.Q：OCCURS 子句能用在第一层吗A：不能（来源：）Q10) What is the difference between index and subscript? GSA10) Subscript refers to the array occurrence while index is the displacement (in no of bytes) from the beginning of thearray. An index can only be modified using PERFORM, SEARCH & SET. Need to have index for a table in order touse SEARCH, SEARCH ALL.Q：索引和下标有什么区别A：下标可以指定数组中任意中位置的元素（只要知道其下标），下标只能是数字型常量或者数字型变量（但是不能在指定的时候修改，如：A（K+1）这样是不行的，要修改的话要在指定的外部改，如：ADD 1 TO K，而索引的话是从表头/数组头开始检索（以BY N的指定检索规律往后滚）再者，索引只能通过PERFORM, SEARCH 和SET来修改，如果要在一个表中使用SEARCH, SEARCH ALL，那这个表就要有索引（因为SEARCH, SEARCH ALL的参数中指定索引，所以即使其有很多限制还是得用它）Q11) What is the difference between SEARCH and SEARCH ALL? GSA11) SEARCH is a serial search.SEARCH ALL is a binary search & the table must be sorted ( ASCENDING/DESCENDING KEY clause to be used & data loaded in this order) before using SEARCH ALL.Q：SERACH和SERACH ALL有什么区别A：SEARCH是顺序查找SERACH ALL 是2叉查找（相信数据结构学过2叉树的都不会陌生），在使用SEARCH ALL前表必须有一个递增/递减的KEY，并且表已经按照其KEY值排序了，这样才能使用SEARCH ALLQ12) What should be the sorting order for SEARCH ALL? GSA12) It can be either ASCENDING or DESCENDING. ASCENDING is default. If you want the search to be done on anarray sorted in descending order, then while defining the array, you should give DESCENDING KEY clause. (Youmust load the table in the specified order).Q：为了使用SEARCH ALL，存贮顺序是怎么样的A：他必须是递增或者是递减的，默认地政。如果你想在一个递减顺序存贮的表/数组使用搜索的话，那么当定义表/数组的时候你应该加一个DESCENDING KEY子句（这之前表要已经按指定的顺序排序了）Q13) What is binary search?A13) Search on a sorted array. Compare the item to be searched with the item at the center. If it matches, fine else repeat the process with the left half or the right half depending on where the item lies.Q：什么是2叉查找A：将你要找的目标项与数组的正中项比较，找到就结束搜索，没找到则继续如此循环（比较下一个中值），取哪一半取决于目标值大于中值还是小于中值PS：联想2叉树的查找规律就很好理解，因为所谓的“表“本身也就是数组Q14) My program has an array defined to have 10 items. Due to a bug, I find that even if the program access the11th item in this array, the program does not abend. What is wrong with it?A14) Must use compiler option SSRANGE if you want array bounds checking. Default is NOSSRANGE.Q：我的程序有个数组定义了10项。因为有个BUG，我发现即使访问第11项，程序也不异常终止。那是出了什么问题A：必须使用编译器的一个选项SSRANGE，如果你想检查数组的超界问题。默认是NOSSRANGEQ15) How do you sort in a COBOL program? Give sort file definition, sort statement syntax and meaning. GSA15) Syntax: SORT file-1 ON ASCENDING/DESCENDING KEY key. USING file-2 GIVING file-3.USING can be substituted by INPUT PROCEDURE IS para-1 THRU para-2GIVING can be substituted by OUTPUT PROCEDURE IS para-1 THRU para-2.file-1 is the sort (work) file and must be described using SD entry in FILE SECTION.file-2 is the input file for the SORT and must be described using an FD entry in FILE SECTION and SELECTclause in FILE CONTROL.file-3 is the out file from the SORT and must be described using an FD entry in FILE SECTION and SELECTclause in FILE CONTROL.file-1, file-2 & file-3 should not be opened explicitly.INPUT PROCEDURE is executed before the sort and records must be RELEASEd to the sort work file from the input procedure.OUTPUT PROCEDURE is executed after all records have been sorted. Records from the sort work file must be RETURNed one at a time to the output procedure.Q：怎么在一个COBOL程序中排序？给出排序文件的定义，排序语法和意思A：语法就是SORT file-1 ON ASCENDING/DESCENDING KEY key. USING file-2 GIVING file-3.USING后程序的输入接口，这个地方可以替换成一个输出过程，也就是说写一个过程往USING这个接口中导数据（要在这个过程中READ，AT END，），这个过程在将数据释放到执行排序的文件中之前执行，GIVING后是输出借口，用法类似。此例中输入文件是file-2输出文件是file3(这样个文件必须在文件区中用FD和在文件控制中用到SELECT)真正执行排序的file-1，这里需要注意的是file-1中的文件区不能用FD，应该用SD，file-2和3还是一样（用FD），具体可以看一下书上的例子Q16) How do you define a sort file in JCL that runs the COBOL program?A16) Use the SORTWK01, SORTWK02,. dd names in the step. Number of sort datasets depends on the volume of databeing sorted, but a minimum of 3 is required.Q：怎么在JCL中定义一个排序文件来跑这个COBOL程序A：用SORTWK01, SORTWK02,.作为DATA SET NAME。用多少取决于你要排序的数量，但是至少3个。Q17) What is the difference between performing a SECTION and a PARAGRAPH? GSA17) Performing a SECTION will cause all the paragraphs that are part of the section, to be performed.Performing a PARAGRAPH will cause only that paragraph to be performed.Q：执行一个区和一个段有什么区别A：简单来说的话就是区的概念比段大，执行一个区就要执行其内部所有段，执行段的话只执行该段。（来源：）Q18) What is the use of EVALUATE statement? GSA18) Evaluate is like a case statement and can be used to replace nested Ifs. The difference between EVALUATE andcase is that no break is required for EVALUATE i.e. control comes out of the EVALUATE as soon as one match ismade.Q：EVALUATE语句有什么作用A：EVALUATE就象个CASE语句（多重开关语句，学过C的总知道吧），不同点在于EVALUATE不需要BREAK，一旦匹配就跳出EVALUATE语句了Q19) What are the different forms of EVALUATE statement?A19)EVALUATE EVALUATE SQLCODE ALSO FILE-STATUSWHEN A=B AND C=D WHEN 100 ALSO 00imperative stmt imperative stmtWHEN (D+X)/Y = 4 WHEN -305 ALSO 32imperative stmt imperative stmtWHEN OTHER WHEN OTHERimperative stmt imperative stmtEND-EVALUATE END-EVALUATEEVALUATE SQLCODE ALSO A=B EVALUATE SQLCODE ALSO TRUEWHEN 100 ALSO TRUE WHEN 100 ALSO A=Bimperative stmt imperative stmtWHEN -305 ALSO FALSE WHEN -305 ALSO (A/C=4)imperative stmt imperative stmtEND-EVALUATE END-EVALUATEQ20) How do you come out of an EVALUATE statement? GSA20) After the execution of one of the when clauses, the control is automatically passed on to the next sentence after theEVALUATE statement. There is no need of any extra code.Q：怎么跳出一条EVALUATE语句A：象18题目说的那样，一旦匹配了某一个“WHEN“语句就自动跳出了，不需要什么额外的代码来跳出Q21) In an EVALUATE statement, can I give a complex condition on a when clause?A21) Yes.Q：在一个EVALUATE语句的某个WHEN分支中能否再插入复杂的情况（也就是嵌套）A：当然可以，当多个参数作为控制变量的时候1个WHEN内部可以嵌套更多的情况Q22) What is a scope terminator? Give examples.A22) Scope terminator is used to mark the end of a verb e.g. EVALUATE, END-EVALUATE; IF, END-IF.Q：什么是结束终止符A：结束终止符是搭配一些范围指令的，也就是标识一些范围指令的结束。如：EVALUATE, END-EVALUATE; IF, END-IF 如果没有该结束符，该条语句将终止不了Q23) How do you do in-line PERFORM? GSA23) PERFORM END-PERFORMQ：怎么使用内嵌的PERFORMA：PERFORM END-PERFORM所谓内嵌也就是PERFORM被嵌在某些比如循环语句中担当执行主体，同时通过UNTIL来指定结束判定Q24) When would you use in-line perform?A24) When the body of the perform will not be used in other paragraphs. If the body of the perform is a generic type of code(used from various other places in the program), it would be better to put the code in a separate Para and usePERFORM Para name rather than in-line perform.Q：什么时候使用内嵌式PERFORMA：当该段PERFORM的内容不被其他段用到，只在某些局部代码中（当然PERFORM的主体所用到的参数也都是局部的，例如循环）使用，如果PERFORM主体的代码是一般的（用到了别的程序段的变量），还是使用PERFORM Para name这样的形式比较好（也就是相对与内于PERFORM的外部PERFORM）。Q25) What is the difference between CONTINUE & NEXT SENTENCE ?A25) They appear to be similar, that is, the control goes to the next sentence in the paragraph. But, Next Sentence wouldtake the control to the sentence after it finds a full stop (.). Check out by writing the following code example, one ifsentence followed by 3 display statements (sorry they appear one line here because of formatting restrictions) If 1 0then next sentence end if display line 1 display line 2. display line 3. * Note- there is a dot (.) only at the end ofthe last 2 statements, see the effect by replacing Next Sentence with Continue *Q：CONTINUE 和 NEXT SENTENCE有什么不同A：两者比较相似，都是将程序控制权交给下一句，但是用NEXT SENTENCE的时候，只有当碰到句结束符（就是句末的.）才会将执行下句这道题我用了2个例子测试了一下：1：IF TEST-NUMERIC 0THEN NEXT SENTENCEEND-IFDISPLAY LINE1 DISPLAY LINE2. DISPLAY LINE3.（请注意代码中的.号）结果输出：LINE32：IF TEST-NUMERIC 0THEN CONTINUEEND-IFDISPLAY LINE1 DISPLAY LINE2. DISPLAY LINE3.结果输出：LINE1LINE2LINE3相信已经区别已经比较明显了，NEXT SENTENCE是靠句末的结束符（也就是.）来判断下一句的，而CONTINUE是通过句头的保留字（这例中是DISPLAY）来判断下一句的Q26) What does EXIT do ?A26) Does nothing ! If used, must be the only sentence within a paragraph.Q：EXIT语句有什么作用A：什么都不做，如果用到的话，肯定是作为一段的唯一的一句话，注意：这里不是子程序中用的EXIT PROGRAME（来源：）Q27) Can I redefine an X(100) field with a field of X(200)?A27) Yes. Redefines just causes both fields to start at the same location. For example:01 WS-TOP PIC X(1)01 WS-TOP-RED REDEFINES WS-TOP PIC X(2).If you MOVE 12 to WS-TOP-RED,DISPLAY WS-TOP will show 1 whileDISPLAY WS-TOP-RED will show 12.Q：能不能把X（100）的区域重定义成X（200）A：可以，重定义只是相当于把两个区域的首地址放在一起，从上面这个例子也很好理解A28) Can I redefine an X(200) field with a field of X(100) ?Q31)1 Yes.Q：能不能把X（200）的区域重定义成X（100）A：可以，原因同上Q31)2 What do you do to resolve SOC-7 error? GSQ31) Basically you need to correcting the offending data. Many times the reason for SOC7 is an un-initialized numeric item.Examine that possibility first. Many installations provide you a dump for run time abends ( it can be generated alsoby calling some subroutines or OS services thru assembly language). These dumps provide the offset of the lastinstruction at which the abend occurred. Examine the compilation output XREF listing to get the verb and the linenumber of the source code at this offset. Then you can look at the source code to find the bug. To get capture theruntime dumps, you will have to define some datasets (SYSABOUT etc ) in the JCL. If none of these are helpful, usejudgement and DISPLAY to localize the source of error. Some installation might have batch program debuggingtools. Use them.Q：怎么解决SOC-7错误A：基本上你要看一下一些比较奇怪的数据，很多导致SOC7的原因都是因为数据项的初始化。首先检查所有的可能性。某些功能可能提供一个空间用来存贮那些运行时间ABEND，并且提供最近一次运行时间ABEND的偏移量的说明（也就是位于队列中的位置），检查编译器的输出XREF队列以获得一些关键字。然后你就能看下源代码找出BUG。为了捕获一些运行时间的信息，你需要在JCL中建一个DATASET（象SYSABOUT这样的），如果这些都没用，那么再审查一下ERROR出现的位置判断一下原因。有些软件安装了会提供批处理程序调试工具，那么可以就可以用这些工具了。PS：以上大意就是说SOC-7这个错误多半是因为数据项初始化造成的，然后你应该到运行后编译器的返回信息中去找这些ERROR出现的地方（我们常用的话应该就是走查LOG），查的时候多注意下数据项的初始化问题。Q32) How is sign stored in Packed Decimal fields and Zoned Decimal fields?Q32) Packed Decimal fields: Sign is stored as a hex value in the last nibble (4 bits ) of the storage.Zoned Decimal fields: As a default, sign is over punched with the numeric value stored in the last bite.Q：在内部十进制区域和显示十进制区域符号是怎么存贮的A：内部十进制是一个数字占4位（半字节），内存中用16进制来存，最后在追加4位作为符号，如-4=01001101（末尾的1101表示负，1100表示正），而我们用于显示的十进制，符号并不占空间，只是在最后一位上标识一下Q33) How is sign stored in a comp-3 field? GSQ33) It is stored in the last nibble. For example if your number is +100, it stores hex 0C in the last byte, hex 1C ifyour number is 101, hex 2C if your number is 102, hex 1D if the number is -101, hex 2D if the number is -102 etcQ：COMP-3区怎么存储符号A：COMP-3采用的是内部十进制的存储方式，所谓内部十进制就是压缩式的外部十进制存储方式，上题讲过外部十进制每个数值都用1个字节存储，但前4位是存符号的，这样比较浪费存储空间，所以内部十进制的存储方式就用半个字节（4位）存储一个数字，在最后增加4位作为符号（1100（C）为正，1101（D）为负）Q34) How is sign stored in a COMP field ? GSQ34) In the most significant bit. Bit is ON if -ve, OFF if +ve.Q：COMP区怎么存储符号A：COMP是采用定点二进制的方式存储数据，也就是将一个十进制的数值转化成二进制再进行存储，因为机器存储的形式也是二进制，所以定点二进制的读取是最快速的，因为COMP型的数据是用做计算（也就是说不用再转化成十进制打印），使用定点二进制将会非常高效。这样的存储方式符号是保存在最高有效果位上，如：10=（00001010）?，-10=（00011010）?（来源：）Q35) What is the difference between COMP & COMP-3 ?Q35) COMP is a binary storage format while COMP-3 is packed decimal format.Q：COMP和COMP-3什么区别A：这之前讲过了，COMP采用定点二进制存储，COMP-3采用内部十进制存储Q36) What is COMP-1? COMP-2?Q36) COMP-1 Single precision floating point. Uses 4 bytes.COMP-2 Double precision floating point. Uses 8 bytes.Q：COMP-1是什么？什么是COMP-2A：其实之所以定义计算型数据（COMPCOMP-3）以区别DISPLAY（能计算，但是要用于打印）是为了考虑效率，因为大家知道文件导入（也就是USER使用的数据）一般是十进制的，而机器存储都是二进制，那么当定义的数据光用来计算不用打印，处于效率考虑会把它定义成COMP型，当然就会衍生出几类COMP以适应不用的数据类型的存储。很明显，这里COMP-1就是采用内部短浮点（4个字节表示一个数，8位指数部分，24位表示数字部分），COMP-2用内部长浮点型（8个字节表示一个书，16位指数部分，48位表示数字部分）以适应浮点数据的存储，长浮点精确度更高.Q37) How do you define a variable of COMP-1? COMP-2?Q37) No picture clause to be given. Example 01 WS-VAR USAGE COMP-1.Q:怎么定义一个COMP-1型？COMP-2型A：不要用PICTURE描述，因为是确定分配多少内存的，直接用USAGE，如01 WS-VAR USAGE COMP-1Q38) How many bytes does a S9(7) COMP-3 field occupy ?Q38) Will take 4 bytes. Sign is stored as hex value in the last nibble. General formula is INT(n/2) + 1), where n=7 in thisexample.Q：一个S9（7）的COMP-3型占用多少字节？A：占用4字节。COMP-3用内部十进制存储，S9（7）中的S是要占空间的，符号占4位，7个数字，每个4位（半个字节），所以是（4+7*4）/8=4字节（字节和位的比例不要搞错了哦）Q39) How many bytes does a S9(7) SIGN TRAILING SEPARATE field occupy ?Q39) Will occupy 8 bytes (one extra byte for sign).Q：一个S9(7) SIGN TRAILING SEPARATE区域占多少字节A：这里是每个符号单独分配空间（也就是没个数值用1个字节表示，就象最常用的DISPLAY型的分配方式），算上S的空间，所以是7+1=8字节。一般情况省略的SIGN子句都是隐含SIGN IS TRAILING的Q40) How many bytes will a S9(8) COMP field occupy ?Q40) 4 bytes.Q： 一个S9(8) COMP 区域占多少字节A：如果之前关于COMP的解释听懂了的话，那很显然就是4字节了（定点二进制用2字节存储14，4字节存储59，类推，你可以自己推下），如果这块还不懂的可以问我或者查下书Q41) What is the maximum value that can be stored in S9(8) COMP?Q41) 99999999Q：S9（8）COMP型最多存储的最大值是什么A：除了8个数值外不要忽略符号位Q42) What is COMP SYNC?Q42) Causes the item to be aligned on natural boundaries. Can be SYNCHRONIZED LEFT or RIGHT. For binary dataitems, the address resolution is faster if they are located at word boundaries in the memory. For example, on mainframe the memory word size is 4 bytes. This means that each word will start from an address divisible by 4. If myfirst variable is x(3) and next one is s9(4) comp, then if you do not specify the SYNC clause, S9(4) COMP will startfrom byte 3 ( assuming that it starts from 0 ). If you specify SYNC, then the binary data item will start from address 4.You might see some wastage of memory, but the access to this computational field is faster.Q：COMP SYNC是什么A：使数据项按“自然边界”排列。SYNCHRONIZED（简写SYNC）语句是同步安置语句。不同的机器会有一个机器字的概念（以一个WORD四个字节举例，这个数字因机器各异，但往往是四个字节），两个机器字之间就是这里说的“自然边界”，也就是说机器每次从内存中取出二个字节长度的数据，但是一个数据项中含有的数值可能跨越几个机器字或者未填满机器字，这样连续读取虽然比较省空间，但是要引用某些机器字的时候要把多个拿出来重新组织（因为一个数值可能跨越多个数据字，也可能未满，机器就要判断一个机器字中哪些是前一个数值哪些是下个数值）。如果向左对齐的话就是想左“自然边界”靠，也就是说未满一个数据字的用空格（对非数字项）或者零（数字项）填充，填充部分不能插入其他数据项的内容，同理向右对齐就是向机器字的右“自然边界”靠，类似的在左边的空余部分填充。按照自然边界存储相当与牺牲空间换取时间，存取效率，系统读取的机器字两段填充区域（0或者SPACE）之间就是一个数值，效率很高。Q43) What is the maximum size of a 01 level item in COBOL I? in COBOL II?Q43) In COBOL II: 16777215Q：COBOL1定义的01层最大大小是多少，COBOL II中？A：COBOL II中是16777215，没什么说的，自己翻书Q44) How do you reference the following file formats from COBOL programs:Q44)Fixed Block File Use ORGANISATION IS SEQUENTIAL. Use RECORDING MODE IS F,BLOCK CONTAINS 0 .Fixed Unblocked Use ORGANISATION IS SEQUENTIAL. Use RECORDING MODE IS F,do not use BLOCK CONTAINSVariable Block File Use ORGANISATION IS SEQUENTIAL. Use RECORDING MODE IS V, BLOCKCONTAINS 0. Do not code the 4 bytes for record length in FD ie JCL rec length will be max rec length in pgm + 4Variable Unblocked Use ORGANISATION IS SEQUENTIAL. Use RECORDING MODE IS V, do not useBLOCK CONTAINS. Do not code 4 bytes for record length in FD ie JCL rec length willbe max rec length in pgm + 4.ESDS VSAM file Use ORGANISATION IS SEQUENTIAL.KSDS VSAM file Use ORGANISATION IS INDEXED, RECORD KEY IS, ALTERNATE RECORD KEY IS RRDS File Use ORGANISATION IS RELATIVE, RELATIVE KEY ISPrinter File Use ORGANISATION IS SEQUENTIAL. Use RECORDING MODE IS F, BLOCKCONTAINS 0. (Use RECFM=FBA in JCL DCB).Q：COBOL中如何涉及（引用调用）以下这些文件A： 文件类型 这里是COBOL在文件控制区中的文件组织访问形式（SELECT下面那句）这里最好翻翻书或者事例代码反复记忆定长文件 用 ORGANISATION IS SEQUENTIAL. Use RECORDING MODE IS F, BLOCK CONTAINS 0固定但是不是以块的组织形式 用 ORGANISATION IS SEQUENTIAL. Use RECORDING MODE IS F, 不要使用BLOCK CONTAINS（因为不是以块的组织形式）变长文件 用 ORGANISATION IS SEQUENTIAL. Use RECORDING MODE IS V，BLOCK ，CONTAINS 0.在之后的文件区中的文件描述FD中不要编码记录长度为4字节变长但是不是以块的组织形式 用 ORGANISATION IS SEQUENTIAL. Use RECORDING MODE IS V，同样不要使用BLOCK CONTAINS也不要编码记录长度为4字节（原因同上）ESDS VSAM文件 用ORGANISATION IS SEQUENTIALKSDS VSAM文件 用ORGANISATION IS INDEXED, RECORD KEY IS, ALTERNATE RECORD作为关键字的RRDS文件 用ORGANISATION IS RELATIVE, RELATIVE KEY IS打印文件 用ORGANISATION IS SEQUENTIAL. Use RECORDING MODE IS F, BLOCK CONTAINS 0. (Use RECFM=FBA in JCL DCB)PS：这道题是阐述COBOL怎么调用外部的各种文件，在文件控制区以及文件区中要定义的一些关键字，有我们最熟悉的FB（定长）和VB（变长）（当然我们的前提是这两类都是以BLOCK（块）为单位的）但是也有不为我们所知的文件类型（可能出现的情况很少，但是也确实存在，比如编译出的MODULE放的LOAD必须是V文件（文件组织形式是V）当然在不写ORGANISATION这些关键字的时候都是默认为FB的，但是在处理一些复杂数据（比如VSAM数据）还有和外部文件（比如JCL）的连接的时候这些保留字都是要指定的，关于更具体的还是要翻书加强记忆Q45) What are different file OPEN modes available in COBOL