[PATCH] Use 32bit memcpy in int1587 when applicable

[Kevin O'Connor]

If the amount of data to be copied is an even number of four, then
copy the data in four byte chunks.  This 32bit copy is more efficient,
in particular when copying to/from memory mapped io.  This should
improve the performance of framebuffer draws in cbvga SeaVGABIOS.

Signed-off-by: Kevin O'Connor 
---
 src/system.c | 32 +++++++++++++++++++-------------
 1 file changed, 19 insertions(+), 13 deletions(-)

diff --git a/src/system.c b/src/system.c
index bcf7e90..60a6fce 100644
--- a/src/system.c
+++ b/src/system.c
@@ -92,8 +92,7 @@ handle_1587(struct bregs *regs)
 // check for access rights of source & dest here

// Initialize GDT descriptor
-    u32 si = regs->si;
-    u64 *gdt_far = (void*)si;
+    u64 *gdt_far = (void*)(regs->si + 0);
     u16 gdt_seg = regs->es;
     u32 loc = (u32)MAKE_FLATPTR(gdt_seg, gdt_far);
     SET_FARVAR(gdt_seg, gdt_far[1], GDT_DATA | GDT_LIMIT((6*sizeof(u64))-1)
@@ -107,10 +106,11 @@ handle_1587(struct bregs *regs)
     loc = (u32)MAKE_FLATPTR(GET_SEG(SS), 0);
     SET_FARVAR(gdt_seg, gdt_far[5], GDT_DATA | lim | GDT_BASE(loc));

-    u16 count = regs->cx;
+    SET_SEG(ES, gdt_seg);
+    u16 count = regs->cx, si = 0, di = 0;
     asm volatile(
         // Load new descriptor tables
-        "  lgdtw %%es:(1<<3)(%%si)\n"
+        "  lgdtw %%es:(1<<3)(%%eax)\n"
         "  lidtw %%cs:pmode_IDT_info\n"

// Enable protected mode
@@ -127,13 +127,14 @@ handle_1587(struct bregs *regs)
         "  movw $(3<<3), %%ax\n" // 3rd descriptor in table, TI=GDT, RPL=00
         "  movw %%ax, %%es\n"

-        // move CX words from DS:SI to ES:DI
-        "  xorw %%si, %%si\n"
-        "  xorw %%di, %%di\n"
-        "  rep movsw\n"
+        // memcpy CX words using 32bit memcpy if applicable
+        "  testw $1, %%cx\n"
+        "  jnz 3f\n"
+        "  shrw $1, %%cx\n"
+        "  rep movsl %%ds:(%%si), %%es:(%%di)\n"

// Restore DS and ES segment limits to 0xffff
-        "  movw $(5<<3), %%ax\n" // 5th descriptor in table (SS)
+        "2:movw $(5<<3), %%ax\n" // 5th descriptor in table (SS)
         "  movw %%ax, %%ds\n"
         "  movw %%ax, %%es\n"

@@ -143,16 +144,21 @@ handle_1587(struct bregs *regs)
         "  movl %%eax, %%cr0\n"

// far jump to flush CPU queue after transition to real mode
-        "  ljmpw $" __stringify(SEG_BIOS) ", $2f\n"
+        "  ljmpw $" __stringify(SEG_BIOS) ", $4f\n"
+
+        // Slower 16bit copy method
+        "3:rep movsw %%ds:(%%si), %%es:(%%di)\n"
+        "  jmp 2b\n"

// restore IDT to normal real-mode defaults
-        "2:lidtw %%cs:rmode_IDT_info\n"
+        "4:lidtw %%cs:rmode_IDT_info\n"

// Restore %ds (from %ss)
         "  movw %%ss, %%ax\n"
         "  movw %%ax, %%ds\n"
-        : "+c"(count), "+S"(si), "+m" (__segment_ES)
-        : : "eax", "di", "cc");
+        : "+a" (gdt_far), "+c"(count), "+m" (__segment_ES)
+        : "S" (si), "D" (di)
+        : "cc");

set_a20(prev_a20_enable);