In September 1975, IBM announced its smallest and first portable computer (If you consider a 28 Kgs. computer portable, that is), the IBM 5100, no bigger than one of IBM's typewriters.
Developed in Rochester, it used the same operating system as IBM's /370 line of main frames. Thus it could accommodate the same APL interpreter, permitting the use of APL programs. A BASIC interpreter was also available, depending of the 5100 version chosen.
This was the first widely marketed and supported personal computer, and definitely the first useful all-in-one, portable computer system. However, it was a very primitive machine that was largely unsuccessful due to its high price tag (basic version costed $8,975) and limited expansion capabilities.
It had a built-in tape drive and a small 5" 64 character display. A special display mode allowed the user to select right or left bigger 32 chars. of each line.
The tape drive used a 1/4 inch DC300 tape cartridge and stored 204 KB of data.
The 5100 didn't feature a microprocessor chip, but a card called PALM (Put All Logic in Microcode) which acted as a 16-bit microprocessor.
Notice that the 5100 is the first serial number of IBM "Personal Computer" range that will later include the 5110, 5120, 5150 (IBM-PC) and 5160 (PC-XT).