Mainframe 技术面试问答

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"Mainframe Q&A涵盖了CICS、COBOL、JCL、VASM和DB2等关键领域的常见问题与解答，旨在帮助理解和掌握大型机技术的核心概念。" 在IT行业中，大型机（Mainframe）是处理海量数据和关键业务系统的核心平台，涉及到的技术包括CICS（Customer Information Control System）、COBOL编程语言、JCL（Job Control Language）、VASM（Virtual Storage Access Method）以及数据库管理系统DB2。 1. **COBOL**：COBOL（Common Business Oriented Language）是一种面向业务的编程语言，广泛用于处理财务、行政和数据处理任务。Q&A中提到的COBOL程序的组成部分包括识别 division、环境 division、数据 division 和过程 division。COBOL的数据类型主要有字母数字（X）、字母（A）和数字（9）。初始化（INITIALIZE）动词用于设置字段的默认值，77级用于定义基本级元素，不能是其他项的子项；88级用于定义条件名，而66级则用于重命名（RENAMES）条款。 2. **CICS**：CICS是IBM开发的一个事务处理监控系统，它允许用户通过终端或网络接口进行实时交互。CICS主要用于大型机上的在线交易处理，如银行、航空公司的预订系统等。 3. **SQL**：SQL是结构化查询语言，用于管理和操作数据库。在Mainframe环境中，SQL常用于与DB2的交互，执行查询、更新和数据管理操作。 4. **DB2**：DB2是IBM的数据库管理系统，尤其适合处理大量并发交易和复杂查询。在Mainframe上，DB2通常用于存储和检索关键业务数据。 5. **IMS**：Information Management System，是IBM的另一个大型数据管理系统，专为处理大规模的层次结构数据而设计，适用于企业级应用，如订单处理和库存控制。 6. **JCL**：JCL是Job Control Language的缩写，是IBM Mainframe系统中用于提交作业和控制作业流程的语言。它定义了作业的执行顺序、资源分配、输入/输出处理等。 7. **VASM**：Virtual Storage Access Method是IBM Mainframe的一种存储管理技术，提供动态地址转换，允许程序访问超过物理内存的虚拟内存空间。这些知识是Mainframe领域专业人士必须掌握的基础，涵盖从编程语言到系统管理的各个方面。理解并熟练运用这些技术对于在大型机环境下进行高效、稳定和安全的操作至关重要。在面试或工作中，熟悉这些Q&A将极大地提升解决问题的能力。

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A73) Scope terminators are mandatory for in-line PERFORMS and EVALUATE statements. For

readability, it's recommended coding practice to always make scope terminators explicit.

Q74) In a COBOL II PERFORM statement, when is the conditional tested, before or after the

perform execution?

A74) In COBOL II the optional clause WITH TEST BEFORE or WITH TEST AFTER can be added to

all perform statements. By default the test is performed before the perform.

Q75) In an EVALaUTE statement is the order of the WHEN clauses significant?

A75) Absolutely. Evaluation of the WHEN clauses proceeds from top to bottom and their sequence can

determine results.

Q76) What is the default value(s) for an INITIALIZE and what keyword allows for an override of

the default.

A76) INITIALIZE moves spaces to alphabetic fields and zeros to alphanumeric fields. The

REPLACING option can be used to override these defaults.

Q77) What is SET TO TRUE all about, anyway?

A77) In COBOL II the 88 levels can be set rather than moving their associated values to the related data

item. (Web note: This change is not one of COBOL II's better specifications.)

Q78) What is LENGTH in COBOL II?

A78) LENGTH acts like a special register to tell the length of a group or elementary item.

Q79) What is the difference between a binary search and a sequential search? What are the

pertinent COBOL

commands?

A79) In a binary search the table element key values must be in ascending or descending sequence. The

table is 'halved' to search for equal to, greater than or less than conditions until the element is

found. In a sequential search the table is searched from top to bottom, so (ironically) the elements

do not have to be in a specific sequence. The binary search is much faster for larger tables, while

sequential works well with smaller ones. SEARCH ALL is used for binary searches; SEARCH for

sequential.

Q80) What is the point of the REPLACING option of a copy statement?

A80) REPLACING allows for the same copy to be used more than once in the same code by changing

the replace value.

Q81) What will happen if you code GO BACK instead of STOP RUN in a stand alone COBOL

program i.e. a

program which is not calling any other program.

A81) The program will go in an infinite loop.

Q82) How can I tell if a module is being called DYNAMICALLY or STATICALLY?

A82) The ONLY way is to look at the output of the linkage editor (IEWL)or the load module itself. If

the module is being called DYNAMICALLY then it will not exist in the main module, if it is

being called STATICALLY then it will be seen in the load module. Calling a working storage

variable, containing a program name, does not make a DYNAMIC call. This type of calling is

known as IMPLICITE calling as the name of the module is implied by the contents of the working

storage variable. Calling a program name literal (CALL

Q83) What is the difference between a DYNAMIC and STATIC call in COBOL.

A83) To correct an earlier answer: All called modules cannot run standalone if they require program

variables passed to them via the LINKAGE section. DYNAMICally called modules are those that

are not bound with the calling program at link edit time (IEWL for IBM) and so are loaded from

the program library (joblib or steplib) associated with the job. For DYNAMIC calling of a module

IBMMAINFRAMES.com

the DYNAM compiler option must be chosen, else the linkage editor will not generate an

executable as it will expect u address resolution of all called modules. A STATICally called

module is one that is bound with the calling module at link edit, and therefore becomes part of the

executable load module.

Q84) How may divisions are there in JCL-COBOL?

A84) Four

Q85) What is the purpose of Identification Division?

A85) Documentation.

Q86) What is the difference between PIC 9.99 and 9v99?

A86) PIC 9.99 is a FOUR-POSITION field that actually contains a decimal point where as PIC 9v99 is

THREE- POSITION numeric field with implied or assumed decimal position.

Q87) what is Pic 9v99 Indicates?

A87) PICTURE 9v99 is a three position Numeric field with an implied or assumed decimal point after

the first position; the v means an implied decimal point.

Q88) What guidelines should be followed to write a structured Cobol prg'm?

A88)

1) use 'evaluate' stmt for constructing cases.

2) use scope terminators for nesting.

3) use in line perform stmt for writing 'do ' constructions.

4) use test before and test after in the perform stmt for writing do-while constructions.

Q89) Read the following code. 01 ws-n pic 9(2) value zero. a-para move 5 to ws-n. perform b-para

ws-n times. b-para.

move 10 to ws-n. how many times will b-para be executed ?

A89) 5 times only. it will not take the value 10 that is initialized in the loop.

Q90) What is the difference between SEARCH and SEARCH ALL? What is more efficient?

A90) SEARCH is a sequential search from the beginning of the table. SEARCH ALL is a binary search,

continually dividing the table in two halves until a match is found. SEARCH ALL is more

efficient for tables larger than 70 items.

Q91) What are some examples of command terminators?

A91) END-IF, END-EVALUATE

Q92) What care has to be taken to force program to execute above 16 Meg line?

A92) Make sure that link option is AMODE=31 and RMODE=ANY. Compile option should never have

SIZE(MAX). BUFSIZE can be 2K, efficient enough.

Q93) How do you submit JCL via a Cobol program?

A93) Use a file //dd1 DD sysout=(*, intrdr)write your JCL to this file. Pl some on try this out.

Q94) How to execute a set of JCL statements from a COBOL program

A94) Using EXEC CICS SPOOL WRITE(var-name) END-EXEC command. var-name is a COBOL host

structure containing JCL statements.

Q95) Give some advantages of REDEFINES clause.

A95)

1. You can REDEFINE a Variable from one PICTURE class to another PICTURE class by

using the same memory

location.

2. By REDEFINES we can INITIALISE the variable in WORKING-STORAGE Section itself.

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3. We can REDEFINE a Single Variable into so many sub variables. (This facility is very useful

in solving Y2000

Problem.)

Q96) What is the difference between static call & Dynamic call

A96) In the case of Static call, the called program is a stand-alone program, it is an executable program.

During run time we can call it in our called program. As about Dynamic call, the called program is

not an executable program it can executed through the called program

Q97) What do you feel makes a good program?

A97) A program that follows a top down approach. It is also one that other programmers or users can

follow logically and is easy to read and understand.

Q98) How do you code Cobol to access a parameter that has been defined in JCL? And do you code

the PARM

parameter on the EXEC line in JCL?

A98)

1) using JCL with sysin. //sysin dd *here u code the parameters(value) to pass in to cobol

program /* and in program

you use accept variable name(one accept will read one row)/.another way.

2) in jcl using parm statement ex: in exec statement parm='john','david' in cobol pgm u have to

code linkage section in that for first value you code length variable and variable name say, abc

pic x(4).it will take john inside to read next value u have to code another variable in the same

way above mentioned.

Q99) Why do we code S9(4) comp. Inspite of knowing comp-3 will occupy less space.

A99) Here s9(4)comp is small integer ,so two words equal to 1 byte so totally it will occupy 2 bytes(4

words).here in s9(4) comp-3 as one word is equal to 1/2 byte.4 words equal to 2 bytes and sign will

occupy 1/2 byte so totally it will occupy 3 bytes.

Q100) The maximum number of dimensions that an array can have in COBOL-85 is ----------- ?

A100) SEVEN in COBOL - 85 and THREE in COBOL - 84

Q101) How do you declare a host variable (in COBOL) for an attribute named Emp-Name of type

VARCHAR(25) ?

A101)

01 EMP-GRP.

49 E-LEN PIC S9(4) COMP.

49 E-NAME PIC X(25).

Q102) What is Comm?

A102) COMM - HALF WORD BINARY

Q103) Differentiate COBOL and COBOL-II. (Most of our programs are written in COBOLII, so,

it is good to know,

how, this is different from COBOL)

A103) The following features are available with VS COBOL II:

1. MVS/XA and MVS/ESA support The compiler and the object programs it produces can be

run in either

24- or 31-bit addressing mode.

2. VM/XA and VM/ESA support The compiler and the object programs it produces can be

run in either

24- or 31-bit addressing mode.

3. VSE/ESA support The compiler and the object programs it produces can be run under

VSE/ESA.

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