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Simple As Possible computer - 1 (SAP-1)

• Introduction
• The computer
  • Debounce circuit
• SAP-1 with no Input switches
• Simulation with GHDL

Introduction

This is a VHDL implementation of the well known computer SAP-1, described in the book Malvino - Digital Computer Electronics - 3rd Edition. The Figure 1 shows the block diagram of the computational unit, where the active states of the signals were chosen to match the ones described in the book. Therefore, the same waveforms presented in the SAP-1 chapter can be visualized in the simulation.

Figure 1: Block diagram of SAP-1 with data entries. Beside each signal, there is how its name is referred into the VHDL code.

The computer

As shown in Figure 2, the computer has eight inputs: the switches s1 to s7, described in Table 1, and the clock input in_clk. The program is loaded into the 8 vs 16 bits RAM memory before the computer run, using the switches s1, s3 and s4. While s5 is set to 0 (clear) and s2 is set to 0 (prog), data and its destination address are fed respectively to s3 and s1 and a pulse in s4 is performed to write the information. The operation is repeated until all the program is recorded, then s2 is set to 0 (run), connecting the Memory Address Register (MAR) to the W bus.

If s7 is 0 (auto), the program starts when s5 is 1 (start) and the computer will run until it finds the instruction HLT. Otherwise, if s7 is 1 (manual), the clock must be manually provided by pressing s6 repeatedly.

Figure 2: Inputs and outputs of this SAP-1 implementation.

Table 1: SAP-1 Switches

Switch	Function
s1	Memory Address
s2	'1' (run): Connects the MAR input to the W bus - '0' (program): Connects the MAR input to s1
s3	Input of data
s4	'1' (read): Memory is ready be read by the SAP-1 - '0' (write): write to the RAM the content of s3 in the address specified by s1
s5	'1' (start): Puts clr and bar_clr signals to the inactive states, starting the computer - 0 (clear): Resets the Program Counter to 0, the Ring Counter to the T1 state and Instruction Register to '00000000'
s6	Single step
s7	'1' (manual): Clock is provided by successively pushing s6 - '0' (auto): clock is read from in_clk

Debounce circuit

A debounce circuit was implemented in the file debounce.vhd and instantiated to the switches s2, s4, s5, s6, s7. To filter the ripple of a commutation, it monitors the state of a switch and if it changes, the circuit stores the value and waits for three clock cycles, then read the input again, if the value remains the same, then the new input is passed on.

In order to reduce simulation time, the amount of clock cycles the debounce circuit waits is 3, which for the current simulation frequency (100 MHz) it leads a delay of 30 ns. However, in a realistic scenario this delay should be around 10 ms, which can be achieved by changing the constant

constant debounce_ticks: integer := 3;

in the isap1.vhd file, accordingly to the selected clock.

SAP-1 with no Input switches

This implementation is mainly intended to provide a way to see the internal signals of SAP-1 during an execution of a program. Therefore, a version with no inputs is also provided, where the simulation is performed without the recording step. The program is written direct by the user to the RAM file ram.vhd and the start of the execution is controlled with the s5 switch.

The Figure 3 shows the block diagram with the Memory Address Register (imar.vhd) and RAM (iram.vhd) replaced by their version with no input, mar.vhd and ram.vhd, respectively. The Figure 4 presents the resulting simplified version of the SAP-1.

Figure 3: Structural model of SAP-1 without the input switches.

Figure 4: SAP-1 With just the start/clear switch.

Simulation with GHDL

The simulation is performed with the free software GHDL, which uses the *.vhd files to generate executables that can be run by your PC and provide waveform outputs. If you are in Linux, make sure to have git, make and GHDL installed and run:

git clone https://github.com/waldeir/SAP-1
cd SAP-1/
make

That will produce the executables isap1_tb and sap1_tb. The first is the simulation for the SAP-1 in Figure 2, the latter is for the version with no input in Figure 4. You can obtain the waveforms vs time graphs of either simulation by running in linux command line:

./sap_tb --vcd=waveform.vcd

or

./isap_tb --vcd=waveform.vcd

The procedure generates the waveform file waveform.vcd, that can be opened in a program like gtkwave, as in Figure 5.

Figure 5: Waveforms of a SAP-1 simulation.

Custom programs can be written to the testbench file isap1_tb.vhd, where they will be loaded to the SAP-1's RAM and then executed.