Red screen with as simple a Makefile that I could manage.

.gitignore

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+*.o
+*.d
+
+*.elf
+*.elf.map
+*.gba
+
+*.sav

scratch/Makefile

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+.SUFFIXES:
+
+# --------------------------------------------------------------------------------------------------
+
+ifeq ($(strip $(DEVKITARM)),)
+$(error "Please set DEVKITARM in your environment. export DEVKITARM=<path to>devkitARM)
+endif
+
+include ${DEVKITARM}/gba_rules
+
+# --------------------------------------------------------------------------------------------------
+
+OUTPUT = scratch
+OFILES = scratch.o
+
+INCLUDE = -I${LIBGBA}/include
+OPTIMISE = -fomit-frame-pointer -ffast-math
+ARCH = -mthumb -mthumb-interwork
+CFLAGS = -g -Wall -O3 -mcpu=arm7tdmi -mtune=arm7tdmi ${OPTIMISE} ${ARCH} ${INCLUDE}
+
+LD := ${CC}
+LDFLAGS = -g ${ARCH} -Wl,-Map,$(notdir $@).map
+LIBPATHS=-L${LIBGBA}/lib
+LIBS := -lgba
+
+DEPSDIR=${CURDIR}
+
+# --------------------------------------------------------------------------------------------------
+
+${OUTPUT}.gba : ${OUTPUT}.elf
+
+${OUTPUT}.elf : ${OFILES}
+
+all : ${OUTPUT}.gba
+
+clean :
+	rm -f *.o *.d *.elf* *.gba
+
+# --------------------------------------------------------------------------------------------------

scratch/scratch.c

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+#include <gba.h>
+
+#define SCREENBUFFER       ((volatile uint16_t*)0x06000000)
+
+int main() {
+  REG_DISPCNT = MODE_3 | BG2_ENABLE;
+
+  for (int pix_idx = 0; pix_idx < SCREEN_WIDTH * SCREEN_HEIGHT; ++pix_idx) {
+    SCREENBUFFER[pix_idx] = 0x001F;
+  }
+
+  while(1) {}
+
+  return 0;
+}