# 25-subprogram

## 电信业务场景

计费子程序。通过CALL语句调用计费计算子程序，传入用量和单价参数，返回计费金额。演示USING参数传递和RETURN-CODE使用。

## Purpose
Tests COBOL subprogram calling conventions including CALL with literal names,
CALL with variable program names, and CALL with IS INITIAL.

## Architecture
- **callee.cbl** — Subprogram (PROGRAM-ID. callee) that adds two numbers via
  LINKAGE SECTION parameters and tracks first-call state.
- **caller.cbl** — Main program (PROGRAM-ID. caller) with three test cases.

## Tests
1. **CALL literal**: CALL "callee" USING 100 200 -> expects 300
2. **CALL variable**: CALL WS-PGM-NAME USING 10 20 -> expects 30
3. **CALL IS INITIAL**: CALL "callee" IS INITIAL USING 1 2 -> expects 3,
   forces subprogram reinitialization

## Key Techniques
- CALL USING for parameter passing
- LINKAGE SECTION for parameter definitions
- GOBACK to return to caller
- RETURN-CODE for status communication
- IS INITIAL to reinitialize subprogram state