Saving some gates by making registers write only.

markusC64 · Sep 10, 2021 · a904294 · a904294
1 parent 0fbc1fe
commit a904294
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Showing 3 changed files with 13 additions and 7 deletions.
diff --git a/fpga/io/acia/vhdl_source/acia6551.vhd b/fpga/io/acia/vhdl_source/acia6551.vhd
@@ -287,11 +287,11 @@ begin
                     io_resp_regs.data(2) <= appl_tx_irq;
                     io_resp_regs.data(3) <= control_change;
                     io_resp_regs.data(4) <= dtr_change;
-                when c_reg_slot_base =>
-                    io_resp_regs.data(6 downto 0) <= std_logic_vector(slot_base);
-                    io_resp_regs.data(7) <= nmi_selected;
-                when c_reg_rx_rate =>
-                    io_resp_regs.data <= std_logic_vector(rx_rate);
+--                when c_reg_slot_base =>
+--                    io_resp_regs.data(6 downto 0) <= std_logic_vector(slot_base);
+--                    io_resp_regs.data(7) <= nmi_selected;
+--                when c_reg_rx_rate =>
+--                    io_resp_regs.data <= std_logic_vector(rx_rate);
                 when others =>
                     null;
                 end case;

diff --git a/software/io/acia/acia.cc b/software/io/acia/acia.cc
@@ -16,6 +16,7 @@ Acia :: Acia(uint32_t base)
     rx_ram = (volatile uint8_t *)(base + ACIA_RX_RAM_OFFSET);
 
     regs->slot_base = 0;
+    slot_base = 0;
     regs->enable = 0;
     controlQueue = NULL;
     dataQueue = NULL;
@@ -45,6 +46,8 @@ int Acia :: init(uint16_t base, bool useNMI, QueueHandle_t controlQueue, QueueHa
     }
     // regs->enable = 0;
     regs->slot_base = (uint8_t)base;
+    slot_base = (uint8_t)base;
+
     printf("ACIA: Wrote %b to the base register.\n", (uint8_t)base);
 
     this->controlQueue = controlQueue;
@@ -199,14 +202,15 @@ void Acia :: AdvanceRx(int adv)
 
 void Acia :: SetRxRate(uint8_t value)
 {
+    // Warning: Write only register
     regs->rx_rate = value;
 }
 
 void Acia :: GetHwMapping(uint8_t& enabled, uint16_t& address, uint8_t& nmi)
 {
     enabled = regs->enable;
-    address = (uint16_t(regs->slot_base & 0x7F) << 2) + 0xDE00;
-    nmi = regs->slot_base >> 7;
+    address = (uint16_t(slot_base & 0x7F) << 2) + 0xDE00;
+    nmi = slot_base >> 7;
 }
 
 // Global Static

diff --git a/software/io/acia/acia.h b/software/io/acia/acia.h
@@ -147,6 +147,8 @@ class Acia
     QueueHandle_t controlQueue;
     QueueHandle_t dataQueue;
     DataBuffer *buffer;
+    uint8_t slot_base; // copy of HW register, as register is now write only
+
     static void TaskStart(void *a);
     void Task();
 public: