Update 2 : The youtube vidoe How did NASA Steer the Saturn V answers this question and then some, a must watch.
Update: I really wanted to know how spacecraft navigation (not guidance) computers worked, given small computational resources. I have asked in another question and edited this question to limit answers to examples of old guidance software source code, For those interested in samples of old guidance software please refer to Samples of old guidance software using computational resources on Earth implementing navigation in space instead. Leaving the original (incorrect question below as it was in order to not make the responses look irrelevant).
In an article I came across the something like "X used hardware programme for Venus mission with 65 KB (not sure if this number is correct?) memory".
I am a software developer and with all the resources available today I cannot fathom where one could even start such an endeavour.
Is there an archive (museum) of old/antique software that was written (hard or soft) for interplanetary missions? if something at a higher level than assembly or the equivalent in today's Java, Pascal, C#, etc. programming languages with no consideration for memory and disk usage then that would be even better.
From what little I understood it seems a task equivalent to construction of pyramids with primitive tools. Are there any simulation or tools to get a today's simpleton programmer a glimpse and appreciation of what those giants did?