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Elektronika MK-85 - external storage

In my opinion, the biggest disadvantage of the MK-85 against the design prototype Casio FX-700P is the lack of an external storage support. The ability of machine code programming makes possible to solve this problem. The proposed project allows to save BASIC programs to a nonvolatile serial memory.


As a mass storage device serves a popular serial EEPROM 93C86 with Microwire bus, connected to the keyboard port. The EEPROM should be inserted or removed only when the calculator is turned off.

Wiring diagram

wiring diagram


Data stored in the EEPROM consist of the BASIC programs address table 822C-823F followed by the code segment starting from address 826A.


  1. Allocate 00B0 bytes of RAM by writing value 8750 to the ramtop system variable 8252, which is equivalent to a string input string typed at the INPUT prompt.
  2. Use the hex loader to write the program code to the allocated area.
  3. The program should be started with a string input string when saving BASIC programs, or with a string input string when loading BASIC programs. Don't press any keys while transfer is in progress. It takes several seconds, depending on the file size. When the transfer is completed, READY P0 should appear on the display.

It is convenient to leave the machine code starter in the program P0 and use only P1-P9 for other BASIC programs.

Program listing

; port 100, bit 7 - Data Output
; port 102, bit 1 - Data Input
; port 102, bit 2 - Clock
; port 102, bit 3 - Chip Select

        .radix  16

start:  mov     #E,@#102        ;CE=0, CK=0, DO=0
        mov     #822C,r4        ;starting RAM address
        mov     #C000,r3        ;READ command and EEPROM address
        clr     r5              ;no delay after transfer
        cmpb    @#825A,#4C      ;'L', load
        beq     loop
        mov     #A000,r3        ;WRITE command and EEPROM address
        mov     #20,r5          ;delay of ca. 10ms after transfer
        cmpb    @#825A,#53      ;'S', save
        bne     end

; Write Enable
        mov     #9800,r1        ;command EWEN
        mov     #E,r2           ;13 (decimal) clock cycles
        jsr     pc,xfer
        mov     #E,@#102        ;CE=0, CK=0, DO=0
; transfer data
loop:   mov     r3,r1
        add     #8,r3           ;increment EEPROM address
        mov     #E,r2           ;13 (decimal) clock cycles
        jsr     pc,xfer
        mov     (r4),r1         ;data to save, ignored when loading
        mov     #11,r2          ;16 (decimal) clock cycles
        jsr     pc,xfer
        mov     #E,@#102        ;CE=0, CK=0, DO=0
        mov     r5,r0           ;delay after transfer
        beq     load
        jsr     pc,@#10FC       ;EEPROM write time of ca. 10ms
        br      next
load:   mov     r1,(r4)         ;loaded data
next:   cmpb    (r4)+,(r4)+     ;advance the RAM address
        cmp     r4,#8240
        bcs     loop
        bne     next1
        mov     #826A,r4        ;skip system variables
next1:  cmp     r4,@#823E
        bcs     loop
; Write Disable
end:    mov     #8000,r1        ;command EWDS
        mov     #E,r2           ;13 (decimal) clock cycles
        jsr     pc,xfer
        mov     #E,@#102        ;CE=0, CK=0, DO=0
; system restart
        clr     r0              ;reset vector
        jmp     (r0)

xfer1:  bic     #4,r0           ;CK=1
        mov     r0,@#102
        mov     @#100,r0
        rolb    r0              ;Carry=/DO
; send r1 through DI, and read from DO to r1
; expects number_of_bits+1 in r2
xfer:   rol     r1              ;Carry=/DI, bit 0 of r1 = /DO
        mov     #3,r0           ;CE=1, CK=0, DI=0
        sbc     r0              ;CE=1, CK=0, DI=/Carry
        asl     r0
        mov     r0,@#102
        sob     r2,xfer1
        com     r1
        rts     pc

Loader data

9000 !DF,15,0E,00,02,01,C4,15
9010 !2C,82,C3,15,00,C0,05,0A
9020 !D7,A7,5A,82,4C,00,11,03
9030 !C3,15,00,A0,C5,15,20,00
9040 !D7,A7,5A,82,53,00,28,02
9050 !C1,15,00,98,C2,15,0E,00
9060 !F7,09,68,00,DF,15,0E,00
9070 !02,01,C1,10,C3,65,08,00
9080 !C2,15,0E,00,F7,09,54,00
9090 !01,13,C2,15,11,00,F7,09
9100 !4A,00,DF,15,0E,00,02,01
9110 !40,11,03,03,DF,09,FC,10
9120 !01,01,4C,10,14,A5,17,21
9130 !40,82,E7,87,02,02,C4,15
9140 !6A,82,1F,21,3E,82,E1,87
9150 !C1,15,00,80,C2,15,0E,00
9160 !F7,09,18,00,DF,15,0E,00
9170 !02,01,00,0A,48,00,C0,45
9180 !04,00,1F,10,02,01,C0,17
9190 !00,01,40,8C,41,0C,C0,15
9200 !03,00,80,0B,C0,0C,1F,10
9210 !02,01,8F,7E,41,0A,87,00
9999 END

Conversion utilities - file size: 21kB, sources and executables, DOS and Windows (in a DOS window)


This utility converts a list of BASIC programs in ASCII format to the internal MK-85 data format.

Usage: program1.bas [program2.bas program3.bas ...]


This utility does the opposite to the previous one, i.e. displays the contents of the EEPROM image.

Usage: infile.bin