• ALU with A and B registers

    Patrick LeBoutillier • 11/13/2022 at 19:49 • 0 comments

    The first module I decided to build for the project was the ALU, combined with the A and B registers. I found the ALU simpler to build because it doesn't hold any state.

    I designed an 8-bit ALU that has 2 8-bit inputs (A and B), as well as a carry-in input and an opcode of 4 bits (therefore supporting 16 operations). It outputs an 8-bit result as well as 4 flags (N, V, C, Z).

    Here is a description of the 16 supported operations:

    #define ALU_ADC  0    // Add with carry, for ADC instruction
#define ALU_AND  1    // Logical AND, for AND instruction
#define ALU_BIT  2    // BIT instruction
#define ALU_CMP  3    // CMP, for CMP instructions
#define ALU_DEC  4    // Decrement B
#define ALU_EOR  5    // Logical XOR, for EOR instruction
#define ALU_INC  6    // Increment B
#define ALU_LSR  7    // Logical Shift Right B
#define ALU_ORA  8    // Logial OR, for ORA instruction
#define ALU_ROL  9    // Rotate Left
#define ALU_ROR  10   // Rotate Left
#define ALU_SBC  11   // Subtract with carry, for SBC instruction
#define ALU_PASS 12   // Passthru B. Can be used to set the N and Z flags
#define ALU_ADD  13   // Add without carry, used in addressing modes
#define ALU_SXT  14   // Sign extend, used in relative addressing
#define ALU_ASL  15   // Arithmetic Shift Left

    All of the data-related 6502 flags (i.e N, Z, V, C) are output by the ALU, so in some cases it is necessary to use the ALU_PASS op to get some flags set, even though there is not really a need for an arithmetic or logic operation. I made this design choice to keep things simple and the chip count low.

    To implement the ALU I used 2 SST39SF020A Flash memory chips, that each have 18 address inputs and 8 data outputs. One Flash memory computes the low 4 bits of the result, the other one the high 4 bits. There a few control signals between both of them, to propagate the carry and other data useful for the flags. For example, the low part has a Z output that indicates if the lower 4 bits are zero, which the high part uses, in addition to its own 4-bt result, to compute the real Z flag. 

    Here is a picture of the completed assembly:

    The controls signals and flag outputs are on top using the black connectors, and the green wires at the bottom will connect to the data bus.