1 00:00:00,840 --> 00:00:03,830 Finally, our last instruction! 2 00:00:03,830 --> 00:00:08,050 Branches conditionally transfer control to a specific target instruction. 3 00:00:08,050 --> 00:00:12,610 But we'll also need the ability to compute the address of the target instruction - that 4 00:00:12,610 --> 00:00:17,180 ability is provided by the JMP instruction which simply sets the program counter to value 5 00:00:17,180 --> 00:00:19,030 from register "ra". 6 00:00:19,030 --> 00:00:26,230 Like branches, JMP will write the PC+4 value into to the specified destination register. 7 00:00:26,230 --> 00:00:30,279 This capability is very useful for implementing procedures in Beta code. 8 00:00:30,279 --> 00:00:35,140 Suppose we have a procedure "sqrt" that computes the square root of its argument, which is 9 00:00:35,140 --> 00:00:37,850 passed in, say, R0. 10 00:00:37,850 --> 00:00:41,730 We don't show the code for sqrt on the right, except for the last instruction, which is 11 00:00:41,730 --> 00:00:43,390 a JMP. 12 00:00:43,390 --> 00:00:47,750 On the left we see that the programmer wants to call the sqrt procedure from two different 13 00:00:47,750 --> 00:00:49,630 places in his program. 14 00:00:49,630 --> 00:00:51,899 Let's watch what happensâ€¦ 15 00:00:51,899 --> 00:00:57,940 The first call to the sqrt procedure is implemented by the unconditional branch at location 0x100 16 00:00:57,940 --> 00:00:59,570 in main memory. 17 00:00:59,570 --> 00:01:03,950 The branch target is the first instruction of the sqrt procedure, so execution continues 18 00:01:03,950 --> 00:01:05,299 there. 19 00:01:05,299 --> 00:01:13,729 The BEQ also writes the address of the following instruction (0x104) into its destination register, 20 00:01:13,729 --> 00:01:14,810 R28. 21 00:01:14,810 --> 00:01:20,070 When we reach the end of first procedure call, the JMP instruction loads the value in R28, 22 00:01:20,070 --> 00:01:26,229 which is 0x104, into the PC, so execution continues with the instruction following the 23 00:01:26,229 --> 00:01:28,170 first BEQ. 24 00:01:28,170 --> 00:01:32,920 So we've managed to return from the procedure and continue execution where we left off in 25 00:01:32,920 --> 00:01:35,219 the main program. 26 00:01:35,219 --> 00:01:39,669 When we get to the second call to the sqrt procedure, the sequence of events is the same 27 00:01:39,669 --> 00:01:47,439 as before except that this time R28 contains 0x67C, the address of the instruction following 28 00:01:47,439 --> 00:01:49,630 the second BEQ. 29 00:01:49,630 --> 00:01:55,759 So the second time we reach the end of the sqrt procedure, the JMP sets the PC to 0x67C 30 00:01:55,759 --> 00:02:01,079 and execution resumes with the instruction following the second procedure call. 31 00:02:01,079 --> 00:02:02,299 Neat! 32 00:02:02,299 --> 00:02:06,439 The BEQs and JMP have worked together to implement procedure call and return. 33 00:02:06,439 --> 00:02:12,349 We'll discuss the implementation of procedures in detail in an upcoming lecture. 34 00:02:12,349 --> 00:02:15,549 That wraps up the design of the Beta instruction set architecture. 35 00:02:15,549 --> 00:02:21,000 In summary, the Beta has 32 registers to hold values that can be used as operands for the 36 00:02:21,000 --> 00:02:22,390 ALU. 37 00:02:22,390 --> 00:02:27,390 All other values, along with the binary representation of the program itself, are stored in main 38 00:02:27,390 --> 00:02:28,960 memory. 39 00:02:28,960 --> 00:02:35,769 The Beta supports 32-bit memory addresses and can access values in 2^32 = 4 gigabytes 40 00:02:35,769 --> 00:02:37,890 of main memory. 41 00:02:37,890 --> 00:02:43,489 All Beta memory access refer to 32-bit words, so all addresses will be a multiple of 4 since 42 00:02:43,489 --> 00:02:44,540 there are 4 bytes/word. 43 00:02:44,540 --> 00:02:48,890 The are two instruction formats. 44 00:02:48,890 --> 00:02:53,920 The first specifies an opcode, two source registers and a destination register. 45 00:02:53,920 --> 00:03:00,090 The second replaces the second source register with a 32-bit constant, derived by sign-extending 46 00:03:00,090 --> 00:03:04,239 a 16-bit constant stored in the instruction itself. 47 00:03:04,239 --> 00:03:09,120 There are three classes of instructions: ALU operations, LD and ST for accessing main 48 00:03:09,120 --> 00:03:13,840 memory, and branches and JMPs that change the order of execution. 49 00:03:13,840 --> 00:03:15,099 And that's it! 50 00:03:15,099 --> 00:03:19,510 As we'll see in the next lecture, we'll be able parlay this relatively simple repertoire 51 00:03:19,510 --> 00:03:24,209 of operations into a system that can execute any computation we can specify.