8085 Machinecode at school

Last Updated or created 2022-05-11

Funny story about learning machinelanguage at school.

It was around 1989, and was attending a class Microcomputer Programming in Machine Language.

We where given a problem we had to solve using 8085 machine code.
The machine we had to program this on was a Intel SDK-85, much like below example.

Intel SDK-85

Note it only had a hex keyboard and 7segmented display. You had to punch in the machinecode into memory slots yourself.

Problem we where given was something like searching for certain data in memory.

Normal procedure was :

  • Draw a flow of instructions (Flowchart)
  • Write the machine languages codes
  • Convert those assembly statements into Opcodes the machine could understand
  • Punch in those numbers, run and verify

Most of us knew a lot of opcodes by heart, but some knew all opcodes. And how many bytes where needed. besides that we had to remember jump and return addresses.

So our teacher presented the problem, when he stopped talking, my friend Martin and I when up to our machines … punching buttons.

” Guys .. you can’t expect it to work without writing the program down first!

A few minutes later .. we pressed enter .. and it worked.

A program like above looked like: 
01 2E 2B 21 00 00 79 BE C2 1F C0 CD 19 C0 CA 1E C0 78 BE C2 06 C0 C3 25 C0 2C C2 1E C0 24 C9 CD 19 C0 C3 06 C0 C9

Cut into opcodes:

01 2E 2B
21 00 00
79
BE
C2 1F C0
CD 19 C0
CA 1E C0
78
BE
C2 06 C0
C3 25 C0
2C
C2 1E C0
24
C9
CD 19 C0
C3 06 C0
C9

Some opcodes used 1 byte, others 2 or 3.
C2 1F C0 – means Jump to address C01F when not zero
C9 – means return, go back to a previous CALL statement

Example of machine language which is translated into above

ADDR ; INSTRUCTION
C000 ; LXI B,0X2B2E
C003 ; LXI H,0X0000
C006 ; MOV A,C
C007 ; CMP H
C008 ; JNZ C01F
C00B ; CALL CO19
COOE ; JZ CO1E
ETC
Almost 255 opcodes