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_posts/2016-04-30-the-intel-8080-cpu.md

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+---
+layout: post
+title: The Intel 8080 CPU
+date: 2016-04-30 14:00:00
+permalink: /blog/2016/04/30/
+---
+
+We'd like to introduce [PC8080](/modules/pc8080/), a new 8080-based machine emulator recently added to the
+PCjs Project.
+
+Our first [8080 Test Machine](/devices/pc8080/machine/test/) loads a copy of the
+[8080 Exerciser](https://web.archive.org/web/20151006085348/http://www.idb.me.uk/sunhillow/8080.html)
+(specifically, [8080EX1](/devices/pc8080/rom/test/8080EX1.MAC)) and intercepts the exerciser's CP/M console
+calls so that you can see its progress in the Control Panel window.  It's a "headless" test machine
+(no keyboard or display), so that's all you get.
+
+The good news: PC8080 passes all the 8080 Exerciser tests.  And it doesn't do it by using all sorts of weird
+"flags tables", which most other 8080 emulations seem to fall back on.
+
+Like all the other CPU emulations in the PCjs Project, PC8080 never "calculates" the flags unless/until they are
+actually required, which considerably speeds up arithmetic operations.
+
+Of particular note are the 8080's subtract,
+compare, and decrement operations, which actually perform addition, not subtraction, by using two's complement
+arithmetic in "stages": the first stage (inverting the source operand) occurs *before* the addition, while the second
+stage (incrementing the inverted operand) occurs *after* the addition.  And it appears to be the result of the *first*
+stage, not the second, that determines the state of the Auxiliary Carry flag (AF).
+
+The behavior of the Auxiliary Carry flag (AF) and the associated DAA instruction are probably the most significant
+(and least understood) *arithmetic* differences between the 8080 and all later x86-based CPUs.  Well, there's also
+the fact that the 8080 doesn't provide an Overflow flag (OF).  Internally however, PC8080 retains the ability to
+calculate overflow (since PC8080 was a fork of PCjs), which should be useful when we add Z80 support to PC8080.
+
+On a related note, [Ken Shirriff](http://www.righto.com/) has some fascinating blog posts on the 8085 that also
+provide clues as to how the 8080 likely operates:
+
+* [Inside the ALU of the 8085 microprocessor (January 2013)](http://www.righto.com/2013/01/inside-alu-of-8085-microprocessor.html)
+* [Silicon reverse engineering: The 8085's undocumented flags (February 2013)](http://www.righto.com/2013/02/looking-at-silicon-to-understanding.html)
+* [The 8085's register file reverse engineered (March 2013)](http://www.righto.com/2013/03/register-file-8085.html)
+* [Reverse-engineering the 8085's ALU and its hidden registers (July 2013)](http://www.righto.com/2013/07/reverse-engineering-8085s-alu-and-its.html)
+* [Reverse-engineering the flag circuits in the 8085 processor (July 2013)](http://www.righto.com/2013/07/reverse-engineering-flag-circuits-in.html)
+* [Reverse-engineering the 8085's decimal adjust circuitry (August 2013)](http://www.righto.com/2013/08/reverse-engineering-8085s-decimal.html)
+
+The idea is to make [PC8080](/modules/pc8080/) sufficiently configurable so that it will work with a variety of
+8080-based systems, including those with memory-mapped video displays (like
+[Space Invaders](/devices/pc8080/machine/invaders/)), as well as simpler terminal-based systems, like the CP/M-based
+systems of old.
+
+In fact, as soon as Space Invaders is working, my next planned adaptation is a DEC VT100 terminal emulator (itself
+an 8080-based machine) which can then be "wired up" to other PCjs machine simulations.  This will not be yet-another
+VT100-compatible emulation -- which, like 8080 emulators, has been done to death -- but rather a simulation
+of the original VT100 hardware, building on [Adam Mayer's](https://github.com/phooky) work
+[reverse-engineering the VT100](https://github.com/phooky/VT100-Hax).
+
+*[@jeffpar](http://twitter.com/jeffpar)*  
+*April 30, 2016*