Proprietary multitasking operating system, based on the 8085; basically the other end of the spectrum from CP/M. Included built-in- interrupt-driven I/O (including 4 channels of async), file types (i.e. differentiated between executable, data, index, etc.), built-in ISAM files and a bunch of other stuff. One of these days I'll chat about it if anyone's curious.

Re: passing arguments on the stack. It's an interesting exercise on the 8085 using the "undocumented" instructions. For example, opcode 38H, which is a two-byte instruction would take SP, add the second instruction byte and stick the result in DE. Opcode 28H would do the same, but use HL instead of SP. Another 8085 16-bit operation, opcode D9H, would store HL in the address pointed to by DE. Opcode EDh would load HL from the address pointed to by DE. There were a couple of other 16-bit operations in 8085 not documented. 08H would subtract BC from HL, 10H shifted HL right one place with sign extension. 18H rotated DE left one place through the carry flag (i.e. 17 bit rotate). There were a few other instructions of less interest: a couple of jumps that tested for unsigned overflow and a conditional restart to location 40H (RST 8) if the overflow flag was set.

It was kind of unfortunate that Intel simply didn't leave blanks in the 8080 documentation for the "do nothing" moves; (e.g. MOV A,A; MOV B,B, etc.). It might have freed up a few more spare opcodes for later exploitation.