创建，添加sdram

rt-thread/components/lwp/arch/x86/i386/lwp_gcc.S

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+/*
+ * Copyright (c) 2006-2021, RT-Thread Development Team
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Change Logs:
+ * Date           Author       Notes
+ * 2021-7-14      JasonHu      first version
+ */
+
+#include "rtconfig.h"
+
+.section      .text.lwp
+
+/*
+ * void lwp_switch_to_user(frame);
+ */
+.global lwp_switch_to_user
+lwp_switch_to_user:
+    movl 0x4(%esp), %esp
+    addl $4,%esp    // skip intr no
+    popal
+    popl %gs
+    popl %fs
+    popl %es
+    popl %ds
+    addl $4, %esp   // skip error_code
+    iret    // enter to user mode
+
+.extern arch_syscall_exit
+.global sys_fork
+.global sys_vfork
+.global arch_fork_exit
+sys_fork:
+sys_vfork:
+    jmp _sys_fork
+arch_fork_exit:
+    jmp arch_syscall_exit
+
+.global sys_clone
+.global arch_clone_exit
+sys_clone:
+    jmp _sys_clone
+arch_clone_exit:
+    jmp arch_syscall_exit
+
+/**
+ * rt thread return code
+ */
+.align 4
+.global lwp_thread_return
+lwp_thread_return:
+    movl $1, %eax   // eax = 1, sys_exit
+    movl $0, %ebx
+    int $0x80
+.align 4
+.global lwp_thread_return_end
+lwp_thread_return_end:
+
+#ifdef RT_USING_SIGNALS
+/**
+ * signal return code
+ */
+.align 4
+.global lwp_signal_return
+lwp_signal_return:
+    movl $0xe000, %eax // special syscall id for return code
+    int $0x80
+.align 4
+.global lwp_signal_return_end
+lwp_signal_return_end:
+
+#endif /* RT_USING_SIGNALS */