Avoid long pauses on serial timeout on boot on physical hardware.

The Pintos loader writes status to the serial port as well as the
VGA console, but this doesn't work out so well on at least some
real hardware when nothing is connected to the serial port: each
character can take a second or so to display because the BIOS
waits for a serial timeout each time.  So, on the first serial
timeout, replace the "int $0x14" call by a pair of no-ops.

The rest of the changes are part of the struggle not to exceed
the maximum loader size.

diff --git a/src/threads/loader.S b/src/threads/loader.S
index a5819dd28429080f7e6e5f7b70dd277579d12950..dd87ea1c794bc1759ceca654fc953eaea082d763 100644 (file)
--- a/src/threads/loader.S
+++ b/src/threads/loader.S
@@ -29,11 +29,12 @@
 # Configure serial port so we can report progress without connected VGA.
 # See [IntrList] for details.
        sub %dx, %dx                    # Serial port 0.
-       mov $0x00e3, %ax                # 9600 bps, N-8-1.
+       mov $0xe3, %al                  # 9600 bps, N-8-1.
+                                       # AH is already 0 (Initialize Port).
        int $0x14                       # Destroys AX.
 
        call puts
-       .string "Pintos loader"
+       .string "PiLo"
 
 #### Read the partition table on each system hard disk and scan for a
 #### partition of type 0x20, which is the type that we use for a
@@ -209,8 +210,12 @@ putc:      pusha
 
        mov $0x01, %ah                  # Serial port output service.
        sub %dx, %dx                    # Serial port 0.
-       int $0x14                       # Destroys AH.
+2:     int $0x14                       # Destroys AH.
+       test $0x80, %ah                 # Output timed out?
+       jz 3f
+       movw $0x9090, 2b                # Turn "int $0x14" above into NOPs.
 
+3:
        cmp $'\r', %al
        jne popa_ret
        mov $'\n', %al
@@ -224,8 +229,9 @@ putc:       pusha
 
 read_sector:
        pusha
-       sub %eax, %eax
-       push %eax                       # LBA sector number [32:63]
+       sub %ax, %ax
+       push %ax                        # LBA sector number [48:63]
+       push %ax                        # LBA sector number [32:47]
        push %ebx                       # LBA sector number [0:31]
        push %es                        # Buffer segment
        push %ax                        # Buffer offset (always 0)