Casio FX-850P

The Casio FX-850P is a scientific calculator introduced in 1987 and sold until 1999.

Specifications

2 lines with 32 5×7 characters LCD
*
8 KB RAM
CPU: VLSI at 1.228 MHz. Hitachi HD62002A01
Integrated speaker
Internal slot for memory expansion
Connector with support for RS232 and Centronics
LCD Driver: 2 x HD66100F
2x 3V CR2032 lithium batteries as main power supply
1x 3V CR1220 lithium battery as memory backup

User's manual at http://www.usersmanualguide.com/casio/calculators/fx-850p
The calculator had a BASIC interpreter, MEMO function, a formula library. The built-in 8 kB memory could be expanded using the optional Casio RP-8 or RP-33 RAM expansion modules. An optional Casio FA-6 interface board provided a cassette tape recorder connector, a Centronics printer connector and an RS-232C port. The calculator could print data and listings on any Centronics printer; printing graphics required the Casio FP-100 plotter-printer.
Later, Casio released the FX-880P, which had 32 kB built-in memory. With a RP-33 expansion module, this model could be upgraded to a total of 64 kB.
The memory layout is:

0000-00FF Screen memory
0100-01FF Reserved for internal functions
0200-02FF INPUT Buffer
0300-03FF CALCJMP, VALF Buffer
0400-04FF Reserved for IN/OUT/CALC modes
0500-074A Reserved for internal functions
074B-0752 Reserved for storing the user PASSWORD
0753-175A Reserved for internal functions
175B-175C Vector pointing to MEMO memory start
175D-1FE4 Reserved for internal functions
1FE5-1FFF Vectors pointing to P0-P9 memory start
2000-9FFF User memory
A000-BFFF Repetition of 0000-1FFF
C000-DFFF Repetition of 0000-1FFF
E000-FFFF Repetition of 0000-1FFF

Memory area A000-FFFF was either available as user memory, or would simply repeat the contents of 0000-1FFF
A few glitches are:

POKE 1867,0 → would delete any user PASSWORD
POKE PEEK+256*PEEK,32 → would recover contents of MEMO after a RESET ALL
POKE PEEK+256*PEEK,26 → would hide contents of MEMO, much like a RESET ALL would do, but without losing the programs

The internal function library was programmed in BASIC itself and could be extracted with a BASIC decompiler.
Any function in the library can be executed from a regular BASIC program by using GOTO "LIB0:NNNN" where NNNN is the function number. The command GOTO "LIB0:0400" executes a self-test program.
Characters 252 to 255 were user defined. They could be defined by issuing the command DEF CHR$="HHHHHHHHHH" where n ranges from 252 to 255 and the H's are 10 hexadecimal digits. Every byte defines the pixel pattern for a column. Since a column is 7 pixels high, the least significant bit of every byte is ignored.
The CHR$ would activate a different character set for Katakana and Kanji characters. CHR$ would deactivate Kanji.
Via a serial cable the calculator could connect to a PC or to another Casio FX-850P, allowing the transfer to MEMO and stored programs.