term: ESC[2J, so clear(1) can actually clear

The terminal understood SGR and nothing else, so there was no way to erase the
screen - printing 18 newlines only scrolls the history up out of sight.
term_erase_all blanks the buffer, resets the colours, homes the cursor and
repaints; the CSI parser routes 'J' to it, and clear(1) is four putc calls.

2 files changed, 48 insertions, 0 deletions

diff --git a/src/term.asm b/src/term.asm
index 1ad120f..ea0cff7 100644
--- a/src/term.asm
+++ b/src/term.asm
@@ -877,6 +877,37 @@ update_tile_attr:
     ret
 
 ; -----------------------------------------------------------------------------
+; term_erase_all - blank every cell, home the cursor and repaint. Backs ESC[2J,
+; which is what clear(1) emits; the terminal had no way to erase itself.
+; -----------------------------------------------------------------------------
+term_erase_all::
+    ld hl, TERM_BUF
+    ld de, TROWS * TSTRIDE
+.clr
+    ld a, $20                   ; space
+    ld [hl+], a
+    dec de
+    ld a, d
+    or e
+    jr nz, .clr
+    ld hl, COL_BUF
+    ld de, TROWS * TSTRIDE
+.clrc
+    ld a, 7                     ; default fg (white), default bg
+    ld [hl+], a
+    dec de
+    ld a, d
+    or e
+    jr nz, .clrc
+    xor a
+    ld [wCurRow], a
+    ld [wCurCol], a
+    ld [wViewTop], a
+    call term_redraw
+    call term_write_tilemap
+    ret
+
+; -----------------------------------------------------------------------------
 ; term_redraw - rebuild every tile from the buffer (init / full refresh).
 ; -----------------------------------------------------------------------------
 term_redraw::
@@ -1239,6 +1270,8 @@ term_putc_body:
     ld a, e
     cp 109                      ; 'm' : set graphics rendition
     jr z, .csi_sgr
+    cp 74                       ; 'J' : erase display (clear(1) sends ESC[2J)
+    jr z, .csi_ed
     cp 59                       ; ';' : parameter separator
     jr z, .csi_sep
     xor a                       ; unknown final byte: end the sequence
@@ -1254,6 +1287,11 @@ term_putc_body:
     xor a
     ld [wAnsiState], a
     ret
+.csi_ed:
+    call term_erase_all
+    xor a
+    ld [wAnsiState], a
+    ret
 
 ; -----------------------------------------------------------------------------
 ; apply_sgr - act on the accumulated SGR parameter in wAnsiParam, updating the
diff --git a/usr/clear.c b/usr/clear.c
new file mode 100644
index 0000000..76ee4dc
--- /dev/null
+++ b/usr/clear.c
@@ -0,0 +1,10 @@
+#include "gbos.h"
+/* clear: erase the screen and home the cursor.
+ * Emits ESC[2J, which the terminal understands (see term.asm term_erase_all) -
+ * printing 18 newlines would only scroll the history up, not clear it. */
+void main(void) {
+    putc(27);
+    putc('[');
+    putc('2');
+    putc('J');
+}