The microcontroller is a 16-bit Hitachi/Renesas HD6432653 [128-pin, 19.66 MHz], (H8S/2653) which has 64K mask ROM and 4K SRAM. FWIW, it has a UART port which is accessible via connector CN7. TODO - Sniff TTY, as there are some interesting strings in the ROM, that are not shown on display.
The flash memory has a capacity of 1MB and is organised as 512K x 16. The SP-808 reads data from the ZIP disk in 512 blocks, coincidence?
https://4donline.ihs.com/images/VipMasterIC/IC/RNSA/RNSAS06856/RNSAS06856-1.pdf
The SP-808 firmware, was supplied as 8 midi files.
The following DOS command converts all 8 MIDI files and writes the result to a single BIN file:
for %i in (SP8EX#?.mid) do Rmid2bin.exe infil=%i outfil=SP8EXall.bin