Allocate runtime memory based on scenarios

Previously, the emulator was configured to allocate 4 GiB of memory
during initialization, which made it unsuitable for environments with
limited memory, such as a virtualized Linux guest configured with 4 GiB.
This commit adjusts the memory mapping requirement to 2^{19} bytes,
sufficient for all known benchmarks.

For configurations with SDL enabled, which support video games like Doom
and Quake, the memory allocation is increased to 4 GiB.

We should develop an adaptive memory mapping mechanism to address this
issue effectively, rather than relying solely on static memory mapping
during initialization.

Reported by ztex <ztex030640417@gmail.com> and
            Yen-Fu Chen <qwe661234@gmail.com>.

Close sysprog21#448

Makefile

@@ -98,6 +98,8 @@ $(call set-feature, SDL)
 ifeq ($(call has, SDL), 1)
 OBJS_EXT += syscall_sdl.o
 $(OUT)/syscall_sdl.o: CFLAGS += $(shell sdl2-config --cflags)
+# 4 GiB of memory is required to run video games.
+$(OUT)/main.o: CFLAGS += -DMEM_SIZE=0xFFFFFFFFULL # 2^{32} - 1
 LDFLAGS += $(shell sdl2-config --libs) -pthread
 LDFLAGS += $(shell pkg-config --libs SDL2_mixer)
 endif

src/main.c

@@ -172,9 +172,10 @@ static void dump_test_signature(const char *prog_name)
 #ifndef CYCLE_PER_STEP
 #define CYCLE_PER_STEP 100
 #endif
-/* MEM_SIZE shall be defined on different runtime */
+/* FIXME: MEM_SIZE shall be defined on different runtime */
 #ifndef MEM_SIZE
-#define MEM_SIZE 0xFFFFFFFFULL /* 2^32 - 1 */
+/* Allocate 2^{19} - 1 bytes, which is ample for all known benchmarks. */
+#define MEM_SIZE 0x80000
 #endif
 #define STACK_SIZE 0x1000       /* 4096 */
 #define ARGS_OFFSET_SIZE 0x1000 /* 4096 */