1 files changed, 37 insertions, 25 deletions
diff --git a/kernel/uart.c b/kernel/uart.c
index ba66237..852a085 100644
--- a/kernel/uart.c
+++ b/kernel/uart.c
@@ -1,3 +1,7 @@
+//
+// low-level driver routines for 16550a UART.
+//
+
 #include "types.h"
 #include "param.h"
 #include "memlayout.h"
@@ -6,41 +10,49 @@
 #include "proc.h"
 #include "defs.h"
 
-//
-// qemu -machine virt has a 16550a UART
-// qemu/hw/riscv/virt.c
+// the UART control registers are memory-mapped
+// at address UART0. this macro returns the
+// address of one of the registers.
+#define Reg(reg) ((volatile unsigned char *)(UART0 + reg))
+
+// the registers. some have different meanings for
+// read and write.
 // http://byterunner.com/16550.html
-//
-// caller should lock.
-//
+#define RHR 0 // receive holding register (for input bytes)
+#define THR 0 // transmit holding register (for output bytes)
+#define IER 1 // interrupt enable register
+#define FCR 2 // FIFO control register
+#define ISR 2 // interrupt status register
+#define LCR 3 // line control register
+#define LSR 5 // line status register
 
-// address of one of the registers
-#define R(reg) ((volatile unsigned char *)(UART0 + reg))
+#define ReadReg(reg) (*(Reg(reg)))
+#define WriteReg(reg, v) (*(Reg(reg)) = (v))
 
 void
 uartinit(void)
 {
-  // disable interrupts -- IER
-  *R(1) = 0x00;
+  // disable interrupts.
+  WriteReg(IER, 0x00);
 
-  // special mode to set baud rate
-  *R(3) = 0x80;
+  // special mode to set baud rate.
+  WriteReg(LCR, 0x80);
 
-  // LSB for baud rate of 38.4K
-  *R(0) = 0x03;
+  // LSB for baud rate of 38.4K.
+  WriteReg(0, 0x03);
 
-  // MSB for baud rate of 38.4K
-  *R(1) = 0x00;
+  // MSB for baud rate of 38.4K.
+  WriteReg(1, 0x00);
 
   // leave set-baud mode,
   // and set word length to 8 bits, no parity.
-  *R(3) = 0x03;
+  WriteReg(LCR, 0x03);
 
-  // reset and enable FIFOs -- FCR.
-  *R(2) = 0x07;
+  // reset and enable FIFOs.
+  WriteReg(FCR, 0x07);
 
-  // enable receive interrupts -- IER.
-  *R(1) = 0x01;
+  // enable receive interrupts.
+  WriteReg(IER, 0x01);
 }
 
 // write one output character to the UART.
@@ -48,9 +60,9 @@ void
 uartputc(int c)
 {
   // wait for Transmit Holding Empty to be set in LSR.
-  while((*R(5) & (1 << 5)) == 0)
+  while((ReadReg(LSR) & (1 << 5)) == 0)
     ;
-  *R(0) = c;
+  WriteReg(THR, c);
 }
 
 // read one input character from the UART.
@@ -58,9 +70,9 @@ uartputc(int c)
 int
 uartgetc(void)
 {
-  if(*R(5) & 0x01){
+  if(ReadReg(LSR) & 0x01){
     // input data is ready.
-    return *R(0);
+    return ReadReg(RHR);
   } else {
     return -1;
   }