From 4d752ef67cfebccf22417da9e47e2e94e14c9687 Mon Sep 17 00:00:00 2001
From: Gelthor <gelthor-gitea_sm@havnor.org.uk>
Date: Fri, 4 Sep 2026 15:25:40 +0100
Subject: [PATCH] Added initial draft of globals and errno

---
 errno.asm   | 24 ++++++++++++++++++++++++
 globals.asm | 18 ++++++++++++++++++
 start.asm   |  4 ++++
 3 files changed, 46 insertions(+)
 create mode 100755 errno.asm
 create mode 100755 globals.asm
 create mode 100755 start.asm

diff --git a/errno.asm b/errno.asm
new file mode 100755
index 0000000..6f2bfb3
--- /dev/null
+++ b/errno.asm
@@ -0,0 +1,24 @@
+; Error numbers should be 16 bit ODD numbers, so they can be loaded as immediates
+; and can be checked with:
+;
+;   qcall falible_function
+;   jne falible_ok ; Jump no error
+;       ; handle error
+;   falible_ok:
+;   ; Happy path
+;   ; ...
+; OR
+;   qcall falible_function
+;   je handle_error ; Jump if error
+;   ; Happy path
+;   ; ...
+;   handle_error:
+;       ; handle error
+
+pub const OK                   = 0x0000
+pub const SCREEN_INVALID_MODE  = 0x0001
+pub const SCREEN_INVALID_WIDTH = 0x0003
+pub const SCREEN_FB_TOO_SMALL  = 0x0005
+
+; Extended error codes, these would require loading a 32 bit value.
+pub const EXT_OK              = 0x00000000
\ No newline at end of file
diff --git a/globals.asm b/globals.asm
new file mode 100755
index 0000000..9dc0df4
--- /dev/null
+++ b/globals.asm
@@ -0,0 +1,18 @@
+; Addresses of global variables
+
+; If not double buffering these point to the same buffer
+pub const FB_DISPLAY_PTR     = 16 ; U32 Currently displayed buffer
+pub const FB_DRAW_PTR        = 20 ; U32 Draw to this buffer
+pub const FRAME_BUFFER_SIZE  = 24 ; U32
+pub const PIXEL_WIDTH        = 28 ; U16
+pub const PIXEL_HEIGHT       = 30 ; U16
+
+; Log2 of width in pixels, e.g.
+; * 10 => 1024 * 768
+pub const LOG_WIDTH          = 32 ; U8
+
+pub const BYTES_PER_PIXEL    = 33 ; U8
+; Log2 of bytes per pixel
+; * 0 => 8 bits per pixel
+; * 2 => 32 bits per pixel
+pub const LOG_BPP            = 34 ; U8
diff --git a/start.asm b/start.asm
new file mode 100755
index 0000000..4c7f7f2
--- /dev/null
+++ b/start.asm
@@ -0,0 +1,4 @@
+@0 ; Reserve space for globals
+ 
+jmp 0x100 ; jump over them
+@0x100 ; start of user code
\ No newline at end of file