The HP 9100B is a improved version of HP 9100A, a first Hewlett Packard calculator introduced in 1968. The 9100 is a scientifics desktop calculator with magnetic core memory, printed circuit board ROM, a CRT display and magnetic cards reader/writer. The 9100 has 32 storage registers and 3-levels stack (registers X, Y and Z). The storage registers are splited in two pages. The pages were labeled + and - and + page was assumed if neither was specified for a store or recall. Each page of core has the 16 registers called 0-9 and a-f. The stack and registers can be easy manipulated with many keys. Many mathematical functions are directly available from the keyboard.

The HP 9100 works with floating point BCD (binary coded decimal) aritmetics in a range of 10^-98 to 10⁺⁹⁹ and never shows more digits after the decimal than they were set with a rotary switch at rigt for - even on number entry. If is typed a larger number than the display could handle, the HP 9100 automatically switch to scientific mode. The HP 9100 has two "guard digits" beyond the displayable 10 digit mantissa. This kept the normal floating point errors "out of sight".

The HP 9100B has programming features. A program with max. 392 program steps can be created in RPN. The calculator use line addressing with two digit hexadecimal line numbers. The "IF" statements could check for less than, equal to or greater than and the lines after the if held a line number that the calculator jumped to if the condition was true. A single flag and simplified 5 level subroutine capability is available. The program memory is shared between programs and data with each storage register yielding 14 program steps. The created program is possible to write to a simple magnetic card. The magnetic card uses two tracks, each track has capacity of 196 program steps. The magnetic cards have size like credit cards. The card is inserted into the slot and it is ejected from the same slot during the read or write.

The HP 9100 has ferrite ROM with capacity of 32 kbits program memory and 1.8 kbits of control memory. The RAM called magnetic core memory has capacity of 4416 bits. The ferrite memory has an advantage - the calculator can be switched off and on without losing data or programs.

The HP 9100B has 3-levels stack (registers X, Y and Z). The bottom is "X" register, accessible from keyboard. In middle is "Y" register, called "accumulator". On top is "Z" register called "temporary". The content of the registers can be rolled down or up. The content of "X" and "Y" can be alternated with a key.

Price in 1968 - 1970 was US$ 4,900.