Typedef reg_e

typedef enum logic [3:0] {
    MBOXW  = 4'd0, // Mailbox write register
    MBOXR  = 4'd1, // Mailbox read register
    STATUS = 4'd2, // Mailbox status register
    ERROR  = 4'd3, // Mailbox error register
    WIRQT  = 4'd4, // Write interrupt request threshold register
    RIRQT  = 4'd5, // Read interrupt request threshold register
    IRQS   = 4'd6, // Interrupt request status register
    IRQEN  = 4'd7, // Interrupt request enable register
    IRQP   = 4'd8, // Interrupt request pending register
    CTRL   = 4'd9  // Mailbox control register
  } reg_e;