Uniformized macros: push save all registers, and use only GAS macros, no CPP

bios_color.S

@@ -1,6 +1,11 @@
 /*
 Write a character N times with given color.
 
+Expected output: "bc", where:
+
+- `b` has red foreground, and green background
+- `c` has the default color (gray on black)
+
 TODO: is this the only way? How to set the current color for ah = 0E?
 
 Color codes: https://en.wikipedia.org/wiki/BIOS_color_attributes
@@ -23,9 +28,9 @@ int $0x10
 /*
 The new color is reused only for character that overwrite the writen region.
 
-Cursor is not moved by the previous interrupt, so this produces a colored 'A'.
+Cursor is not moved by the previous interrupt, so this produces a colored 'a'.
 */
-PUTC($0x62)
-PUTC($0x63)
+PUTC $'b
+PUTC $'c
 
 hlt