tf_s0
{ "s0", (void *)offsetof(struct trapframe, tf_s0), db_rw_ddbreg, NULL },
fp = (register_t)&DDB_REGS->tf_s0; /* s0 = fp */
db_read_bytes((db_addr_t)&tf->tf_s0, sizeof(fp), (char *)&fp);
db_read_bytes((db_addr_t)&tf->tf_s0, sizeof(fp), (char *)&fp);
db_read_bytes((db_addr_t)&tf->tf_s0, sizeof(fp), (char *)&fp);
tf->tf_s0 = fb->fb_reg[FB_S0];
tf->tf_s0 = 0; /* S0 (aka frame pointer) */