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README.md

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+========================================================
+A Verilog HDL version of the old MOS 6502 and 65C02 CPUs
+========================================================
+
+Original 6502 core by Arlet Ottens
+
+65C02 extensions by David Banks and Ed Spittles
+
+==========
+6502 Core
+==========
+
+Arlet's original 6502 core (cpu.v) is unchanged.
+
+Note: the 6502/65C02 cores assumes a synchronous memory. This means
+that valid data (DI) is expected on the cycle *after* valid
+address. This allows direct connection to (Xilinx) block RAMs. When
+using asynchronous memory, I suggest registering the address/control
+lines for glitchless output signals.
+
+[Also check out my new 65C02 project](https://github.com/Arlet/verilog-65c02)
+
+Have fun. 
+
+==========
+65C02 Core
+==========
+
+A second core (cpu_65c02.v) has been added, based on Arlet's 6502
+core, with additional 65C02 instructions and addressing modes:
+- PHX, PHY, PLX, PLY
+- BRA
+- INC A, DEC A
+- (zp) addressing mode
+- STZ
+- BIT zpx, absx, imm
+- TSB/TRB
+- JMP (,X)
+- NOPs (optional)
+- 65C02 BCD N/Z flags (optional, disabled)
+
+The Rockwell/WDC specific instructions (RMB/SMB/BBR/BBS/WAI/STP) are
+not currently implemented
+
+The 65C02 core passes the Dormann 6502 test suite, and also passes the
+Dormann 65C02 test suite if the optional support for NOPs and 65C02
+BCD flags is enabled.
+
+It has been tested as a BBC Micro "Matchbox" 65C02 Co Processor, in a
+XC6SLX9-2 FPGA, running at 80MHz using 64KB of internel block RAM. It
+just meets timing at 80MHz in this environment. It successfully runs
+BBC Basic IV and Tube Elite.
+
+============
+Known Issues
+============
+
+The Matchbox Co Processor needed one wait state (via RDY) to be added
+to each ROM access (only needed early in the boot process, as
+eventually everything runs from RAM). The DIHOLD logic did not work
+correctly with a single wait state, and so has been commented out.
+
+I now believe the correct fix is actually just:
+
+always @(posedge clk )
+    if( RDY )
+        DIHOLD <= DI;
+ 
+assign DIMUX = ~RDY ? DIHOLD : DI;