Editing Apollo Guidance Computer (section)

===Central registers===
The AGC had four 16-bit [[processor register|register]]s for general computational use, called the ''central registers'':
* '''A''': The [[accumulator (computing)|accumulator]], for general computation
* '''Z''': The [[program counter]] – the address of the next instruction to be executed
* '''Q''': The remainder from the <code>DV</code> instruction, and the [[return address (computing)|return address]] after <code>TC</code> instructions
* '''LP''': The lower product after <code>MP</code> instructions

There were also four locations in core memory, at addresses 20–23, dubbed ''editing locations'' because whatever was stored there would emerge shifted or rotated by one bit position, except for one that shifted right seven bit positions, to extract one of the seven-bit interpretive op. codes that were packed two to a word. This was common to Block I and Block II AGCs.