When reading IBM documentation or looking at a piece of IBM hardware, you may see a reference to an unfamiliar term: DASD. In this blog post, I’ll explain what DASD means and what other terms you may know it by.
What is a DASD? What are other terms for DASD?
Today, we typically associate the term DASD with IBM mainframes or minicomputers such as the IBM I series or the older AS/400 line. But IBM also used the term on its x86-based server line. They also used it on its personal computer line, back when they still made personal computers.
DASD is an acronym for Direct Access Storage Device. It’s what the rest of the industry called a hard drive, fixed disk, HDD, or Winchester disk. The kind of device that made Seagate and Western Digital famous and Miniscribe notorious. Winchester was IBM’s code name for the technology while they were developing it in the 1970s.
IBM used the DASD terminology in its large computers after releasing the technology, and they continued using it for consistency’s sake when they moved into smaller computers in the 1980s.
So if you’ve worked with IBM servers and noticed an LED on the case that blinks when the hard drive or disk array is in use even though it is labeled DASD, you were on to something.
IBM was pretty insistent on using that phrase. They even used it on their DOS 4.0 packaging in reference to its ability to access fixed disk partitions larger than 32 MB, even though the operating system made no reference to DASD in any of its user interface. DOS called them fixed disks.
Use of the phrase today
Today the DASD term is a curiosity to vintage computer hobbyists. It’s an IBM thing, like Microchannel. But it is still useful to modern day IT professionals who need to communicate with other professionals who specialize in IBM equipment, particularly mainframe. Knowing the DASD terminology and knowing that an AS/400 isn’t a mainframe are two relatively low-friction ways to signal to an IBM specialist that you want to communicate with them and work with them. In my experience, they appreciate the gesture. That’s especially true if you remember and use additional terms as they come up in conversation.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.
Just like RWM that the term supposedly used by IBM for what the rest of the world called RAM.
And IBM was not wrong. But nether the others.
I totally forgot about RWM! Agreed, none of them were wrong. But I remember people arguing about it decades ago.
And then there’s the pronunciation. I always used “dazz-dee” – two syllables. But I also heard an IBM engineer call it “dazd”. One syllable, sounding Germanic or truncated. I didn’t change the way I said it.
I totally sidestepped the pronunciation. I’ve always heard it pronounced with two syllables. But if you pronounce it with one syllable, then you could use it as a synonym for “saved.” “Yeah, I dazzed it.” Or maybe more properly, “Yeah, I dazzed the data set.” A data set, after all, is what IBM calls a file. Though as a longtime Commodore guy, a datasette will always be a tape recorder to me.