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Toplevel VHDL Structural model of a system containing 8051

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-- Model         :  8051 Behavioral Model,
--                  Top Level Block
--
-- File          :  mc8051.vhd   (MicroController 8051)
--
-- Author        :  Michael Mayer ([e-mail]),
--                  Dr. Hardy J. Pottinger, advisor
--                  Department of Electrical Engineering
--                  University of Missouri - Rolla
--
-- Requires      :  pack8051.vhd (Package containing
--                  needed procedures, types, etc.)
--                  uart.vhd (UART peripheral)
--                  (written by Lingfeng Yuan, Prajakta Kurvey)
--                  see also LIBRARY and USE clauses below
--
-- Thanks to     :  Kyle Mitchel for many comments 
--
-- Inspired from :  Sundar Subbarayan
--                  UC Riverside CS 122a (lab 3)
--                  Professor: Dr.Frank Vahid
--                  17th January 1996
--
-- Date Started  :  September 15, 1997
--
-- Features      :  Entire command set
--                  Support for Intel Hex format
--                  Internal program memory (4Kb)
--                  Internal data memory (256 Bytes) - 8052
--                  Supports external prog mem up to 64 Kb
--                  Supports external data mem up to 64 Kb
--                     using MOVX instr. with 16 bit data ptr.
--                  Supports I/O through 4 ports when not using
--                     above features
--                  Interrupts w/ Priority
--                  Choice of level / edge sensitive interrupts
--                  Timer 1 in mode 2 only
--                  Serial tx / rx in mode 1
--
-- Limitations   :  Reset Port does not function
--                  LIMITED PERIPHERALS:
--                  No timer 0
--                  Limited timer 1 (only mode 2)
--                     can't read from timer value TL0    
--                  Limited UART
--
-- REV    DATE       Description
-- -----  --------   ---------------------------------------
-- 1.0    01/17/97   Work from Sundar Subbarayan and
--                   Dr. Frank Vahid
--
-- 2.0    11/04/97   Initial implementation of command
--                   interpreter for Hex Code set.
--
-- 2.1    11/12/97   Changed memory to separate lo and hi mem
--                   and made all access through functions /
--                   procedures / aliases to allow for
--                   distinction between indirect and direct
--                   accessing of upper 128 bytes of data mem
--                   (for 8052 compatibility).
--
-- 2.2    11/21/97   Made program memory access only through
--                   the process get_pmem and its two
--                   signals: pmem_s1_byte and pmem_s4_byte
--                   Added state machine sensitive to xtal which
--                   governs the machine cycles, port & mem
--                   reads, etc.  Built support for external
--                   program memory read in process get_pmem.
--
-- 2.3    12/12/97   Corrected bug in get_pmem - resync to pc
--                   Moved load_program procedure to pack8051
--                   Converted IF..ELSEIF structure to CASE for
--                   decoding of opcodes.  Completed any missing
--                   commands and verified that all 256 were available
--
-- 3.0    12/13/97   Changed port 3 to a single std_logic_vector
--                   Differentiated between commands that read the
--                   port and those that read the latch.
--                   Added output drivers for port3
--
-- 3.1    12/14/97   Modified procedures in main for accessing
--                   data bytes.  All use get_byte_dmem
--                   and set_byte_dmem for any data access, unless
--                   they access it through aliases (e.g. acc <= val)
--
-- 3.1.1  01/26/98   Added condition of ea_n to the program rom load 
--
-- 3.1.2  02/22/98   Corrected handle_sub's advancing of the pc
--                   Corrected JNC to IF cy='0' instead of '1'
--                  
-- 3.1.3  02/24/98   Corrected MOVX's control of Ports 2 & 3.
--
-- 3.2    07/??/98   Corrections from Kyle Mitchell for
--                   0 or L, 1 or H and for initial boot-up
--
-- 4.0    08/30/98   Added serial UART, timer 1 in mode 2, and
--                   the serial interrupt
--
-- 4.1    09/02/98   Added more interrupts.
--
-- 4.2    10/06/98   Corrected MOVX's control of ALE and PSEN_n
--                   Fixed PC increment on several instructions
--
-- 4.3    10/08/98   Added level / edge sensitive interrupt support
--                   Added priority to interrupts (IP)

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