NAME
     hackem - an emulator for the Hack computer featuring (paper) tape I/O

SYNOPSIS
     hackem [-f frequency] [-s factor] [-t file] prog.rom
     rom prog.rom < prog.hack

DESCRIPTION
     This repository contains an emulator for the computer described in "The
     Elements of Computing Systems" and the course "From Nand to Tetris" by
     Nisan and Schocken.

     The emulator expects a raw ROM image in platform byte order that can be
     produced with the supplied 'rom' tool. The tool accepts a superset of
     the .hack file format. Each line of input must contain exactly 16 ASCII-
     encoded binary digits representing an instruction. As an extension,
     whitespace, empty lines, and shell-style comments introduced by a '#'
     character are ignored.

     This implementation is designed to integrate well with the Unix command
     line. It detects idiomatic infinite loops that are usually used to halt
     execution and terminates with the contents of D register as exit status.
     To facilitate data processing, it is extended with three memory-mapped
     I/O devices that are not described in the book:

          -  A punched tape reader bound to standard input.
          -  A tape punch bound to standard output.
          -  An auxiliary printer (using the same interface as the tape
             punch) bound to standard error.

     The screen and keyboard as described by Nisan and Schocken are not
     currently supported. Reads from the keyboard address always return zero.
     The screen memory acts like regular RAM.

     The options are as follows:

     -f frequency
	     Set the simulated clock frequency, in Hertz (cycles per second).
	     The default is 1000.

     -s factor
	     Scale simulation time by the given factor, without changing the
	     clock frequency. Fractional factors are supported. Factors <1
	     speed up execution. A factor of 0 eliminates tick delay, letting
	     the simulation proceed at maximum speed. The default is 1.

     -t file
	     Trace CPU state during execution, written to the given file in
	     tab-separated value (.tsv) format.

TAPE INTERFACES
     The tape reader provides two addresses, mapped to 0x7000 and 0x7001.

     The first address reflects the current value under the read head. It is
     well-defined only when the control word at the second address indicates a
     valid position. A negative value indicates the end of the tape, or no
     tape present at all - corresponding to EOF on standard input.

     Writing to the first address has no effect.

     The second address provides control over the tape feed. Writing 1 to it
     will advance the tape by one position. The result of writing any other
     value is undefined.

     Reading from the second address yields a status word. A negative value
     (bit 15 set) indicates that the value at the read address is valid. All
     other bits are undefined.

     Thus, the procedure to acquire one byte of input is as follows:

          1.  Write 1 to 0x7001.
          2.  Read 0x7001 until it is negative.
          3.  Read 0x7000; if it is negative, indicate the end of input.
          4.  Otherwise, the value read from 0x7000 is the input byte.

     The tape punch provides two addresses, mapped to 0x7002 and 0x7003.

     A write to the first address causes the corresponding byte to be output
     and the tape advanced automatically to the next position. The result is
     only well-defined if the status word at offset 1 indicates readiness.

     The result of reading from the first address is undefined.

     Reading from the second address yields a status word. A negative value
     (bit 15 set) indicates that the tape is in position for new output. This
     is invalid, however, if bit 14 is also set. Bit 14 indicates the end of
     the tape or that no tape is present, corresponding to EOF on standard
     output. All other bits are undefined.

     The result of writing to the status word is undefined.
 
     Thus, the procedure to produce one byte of output is as follows:

          1.  Read 0x7003 until it is negative.
          2.  If bit 14 is set, indicate an error.
          3.  Write the output byte to 0x7002.

     The auxiliary printer, corresponding to standard error, exposes the same
     interface as the tape punch but is mapped at addresses 0x7004 and 0x7005.

BUILDING
     Simply run 'make' to build and 'make test' to execute the supplied tests.

     This implementation targets the Unix environment, specifically OpenBSD.
     Patches that port it to other systems are welcome.

SEE ALSO
     Noam Nisan and Shimon Schocken, The Elements of Computing Systems, second
     edition, MIT Press, 2021.

     https://nand2tetris.org

AUTHORS
     Sven M. Hallberg <pesco@khjk.org>

     Released under the terms of the ISC license.

     The files 'add.hack' and 'max.hack' were derived from similar files
     included with nand2tetris materials. They are distributed under Creative
     Commons Non-Commercial Attribution Share-Alike (CC BY-NC-SA) 3.0 license.