7542 - baremetal: support cursor on a grapheme

So far we've drawn a space implicitly at the cursor. Now I allow drawing
an arbitrary grapheme when drawing the cursor. But the caller has to
specify what to draw. (The alternative would be for layer 103 to
track every single grapheme on screen along with its color and any other
future attributes, just to be able to paint and unpaint the background
for a single character.)

I've modified existing helpers for drawing multiple graphemes to always
clear the final cursor position after they finish drawing. That seems
reasonable for terminal-like applications. Applications that need to
control the screen in a more random-access manner will need to track the
grapheme at the cursor for themselves.

1 files changed, 9 insertions, 4 deletions

diff --git a/baremetal/500text-screen.mu b/baremetal/500text-screen.mu
index b387496f..fbc2ef43 100644
--- a/baremetal/500text-screen.mu
+++ b/baremetal/500text-screen.mu
@@ -112,11 +112,11 @@ fn cursor-position screen: (addr screen) -> _/eax: int, _/ecx: int {
   return *cursor-x-addr, *cursor-y-addr
 }
 
-fn set-cursor-position screen: (addr screen), x: int, y: int {
+fn set-cursor-position screen: (addr screen), x: int, y: int, g: grapheme {
   {
     compare screen, 0
     break-if-!=
-    set-cursor-position-on-real-screen x, y
+    set-cursor-position-on-real-screen x, y, g
     return
   }
   # fake screen
@@ -157,6 +157,11 @@ fn set-cursor-position screen: (addr screen), x: int, y: int {
   dest <- get screen-addr, cursor-y
   src <- copy y
   copy-to *dest, src
+  #
+  var cursor-x/eax: int <- copy 0
+  var cursor-y/ecx: int <- copy 0
+  cursor-x, cursor-y <- cursor-position screen-addr
+  draw-grapheme screen-addr, g, cursor-x, cursor-y, 0  # cursor color not tracked for fake screen
 }
 
 fn clear-screen screen: (addr screen) {
@@ -168,7 +173,7 @@ fn clear-screen screen: (addr screen) {
   }
   # fake screen
   var space/edi: grapheme <- copy 0x20
-  set-cursor-position screen, 0, 0
+  set-cursor-position screen, 0, 0, space
   var screen-addr/esi: (addr screen) <- copy screen
   var y/eax: int <- copy 1
   var height/ecx: (addr int) <- get screen-addr, height
@@ -187,7 +192,7 @@ fn clear-screen screen: (addr screen) {
     y <- increment
     loop
   }
-  set-cursor-position screen, 0, 0
+  set-cursor-position screen, 0, 0, space
 }
 
 # there's no grapheme that guarantees to cover every pixel, so we'll bump down