In current releases of VTAM®, if the GENCB macroinstruction is used to build an ACB and a length is specified in the GENCB that is smaller than needed for the ACB, the GENCB fails with an error return code. Use the SHOWCB or TESTCB macroinstructions to get the correct ACB length at execution time before issuing GENCB. See the description of the GENCB macroinstruction in GENCB—Generate a control block, for information on how to use SHOWCB and TESTCB with GENCB.

If a program uses specific values to refer to an address beyond the ACB (for example, ORG MYACB+92), fields that are inside the ACB could be referenced (or overlaid) unintentionally. Use the IFGACB DSECT to get the ACB length at assembly time.