itself requiring 15 KB. Assembler F can run under either DOS/360 or OS/360 on a system with a 64 KB memory, with the assembler requiring 44 KB. These assemblers are a standard part of OS/360; the version that was generated was specified at system generation (SYSGEN). Model 44 Programming System Assembler "With certain exceptions, the IBM System/360 Model 44 Programming System Assembler Language is a selected subset of the languages available in the IBM System/360 programming support." Most significantly the Model 44 assembler lacked support for macros and continuation statements. On the other hand it had a number of features not found in other System/360 assemblers—notably instructions to update a card image source dataset, named common, and implicit definition of SETA assembler variables. Assembler G "Assembler G" is a set of modifications made to Assembler F in the 1970s by the University of Waterloo (Assembler F was/is open source). Enhancements are mostly in better handling of input/output and improved buffering which speed up assemblies considerably. "Assembler G" was never an IBM product. Assembler H Assembler H Version 2 was announced in 1981 and includes support for Extended Architecture (XA), including the AMODE and RMODE directives. It was withdrawn from marketing in 1994 and support ended in 1995. It was replaced by High Level Assembler. Assembler XF Assembler XF is an upgrade of Assembler F which includes the new System/370 architecture instructions. This version provides a common assembler for OS/VS and DOS/VS systems. Other changes include relaxing restrictions on expressions and macro processing. Assembler XF requires a minimum partition/region size of 64 KB (virtual). Recommended size is 128 KB. High Level Assembler High Level Assembler (HLASM), announced in 1992 as a licensed program "becomes the default translator for System/370 (TM) and System/390 (TM) operating environments." The assembler supports the MVS, VSE, and VM operating systems and successors. As of Release 6 it now is able to run under Linux on IBM Z and generate ELF or GOFF object files. It features a long list of mostly usability enhancements, and incorporates the SLAC (Stanford Linear Accelerator) modifications to Assembler H. Among features added were an indication of CSECT/DSECT for location counter, dependent and labelled USING statements, a list of USING statements currently active, an indication of whether a variable is read or written in the cross-reference, and allowing mixed-case symbol names. The RSECT directive (Read-only Control Section) allows the assembler to check