Machine Cycle- 1: OFMC: Status signals IO/M=0, S 1 =1, S 0 =1

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1 Lecture Rcond: This is a conditional return statement. It is also a part of the subroutine. Whenever this instruction is executed, μp checks the conditional flags. If the condition is found true then the μp returns to the main program by loading the (PC) with the return address stored at the top of the stack. If the condition is found not true then the next instruction of the subroutine will be executed. This is a single byte instruction. The operation code format is 1 1 C C C N There are 8 variations in this statement depending upon CCC. They are RNZ, RZ, RNC, RC, RPO, RPE, RP and RM. The addressing mode is register indirect because the return address is available in the memory location pointed by SP. The macro RTL implemented is, If CCC= TRUE (PCL) M [(SP)] (PCH) M [(SP) +1 ] (SP) (SP) + 2 Else Nothing. where (PCH,PCL) is the return address. If the condition tested is TRUE, then the 8-bit data (lower order 8-bit return address) from the top of the stack pointed by (SP) is moved to (PCL). The 8-bit data (higher order 8-bit return address) from next higher memory location whose address is one more than the content of (SP) is moved to (PCH). The content of the register (SP) is incremented by 2 in this

2 process so that it always point to the top of the stack. Control is transferred back to the main program. If the condition is not found TRUE, the control goes to the next instruction of subroutine in sequence. The macro RTL implementation flowchart is shown below: Machine Cycle- 1: T 1 : A 15 -A 8 (PCH), AD 7 -AD 0 (PCL), ALE = T 2 : RD = 0, (PC) (PC) +1, AD 7 -AD 0 M(AB) T 3 : RD = 1,, (IR) AD 7 -AD 0 T 4 : μp decodes the opcode. Rcond = 1. T 5 : If condition CCC is TRUE then T 6 : go for MC-2 else nothing. Machine Cycle- 2: MRMC: Status signals IO/M=0, S 1 =1, S 0 =0 T 2 : RD = 0, (SP) (SP) +1, AD 7 -AD 0 M(AB) T 3 : RD = 1,, (PCL) AD 7 -AD 0 Machine Cycle- 3: MRMC: Status signals IO/M=0, S 1 =1, S 0 =0 T 2 : RD = 0, (SP) (SP) +1, AD 7 -AD 0 M(AB) T 3 : RD = 1,, (PCH) AD 7 -AD 0

3 Thus, if condition is TRUE it requires 3 machine cycles and 12 states otherwise one machine cycle OFMC and 6 states. For 2MH 3 internal clock the time required is 6μsec or 3 μsec. 7. RST n: This is a single byte unconditional subroutine call instruction. The address of the subroutine is fixed depending upon the decimal number n in the instruction. The decimal number n can change from 0 to 7. The corresponding binary number (NNN) 2 is available as D 5 D 4 D 3 bits of the operation code. N N N n RST-n RST RST RST RST RST RST RST RST-7 The operation code format is 1 1 N N N N There are 8 variations of this instructions depending upon NNN as specified in the table above. The subroutine starting address is calculated as follows: Multiply the decimal number n by 8. Convert it to the corresponding 2 digit hexa decimal number (Y 1 Y 0 ) H (= 8 x n). Append

4 (00) H for the two most significant hexa digits (Y 3 Y 2 ) H so that a 16-bit address (Y 3 Y 2 Y 1 Y 0 ) H (00 Y 1 Y 0 ) H is obtained. E.g. the subroutine starting address for RST-5 is (8 x 5) D = 40 D = 28 H (0028) H. The macro RTL implemented is, M [(SP) -1 ] (PCH) M [(SP) - 2 ] (PCL) (SP) (SP) 2 (PC) (8 x n) where (PCH,PCL) is the return address. The higher order 8-bits of the return address are moved to the memory location whose address is one less than the content of (SP). The lower order 8-bits of the return address are moved to the memory location whose address is two less than the content of (SP). The content of the register (SP) is decremented by 2 in this process so that it always point to the top of the stack. Control is transferred to the instruction whose address is calculated according to number n specified in the instrcution. The addressing mode used is register indirect addressing mode because the return address has to be saved on the top of the stack pointed to by 16-bit register (SP). The macro RTL flow is as shown below: Machine Cycle- 1: T 1 : A 15 -A 8 (PCH), AD 7 -AD 0 (PCL), ALE = T 2 : RD = 0, (PC) (PC) +1, AD 7 -AD 0 M(AB)

5 T 3 : RD = 1,, (IR) AD 7 -AD 0 T 4 : μp decodes the opcode. RST-n = 1. T 5 : Address (8 x n) is calculated and stored in (W,Z) register pair. T 6 : (SP) (SP) -1 Machine Cycle- 2: MWRMC: Status signals IO/M=0, S 1 =1, S 0 =1 T 2 : WR = 0, AD 7 -AD 0 (PCH) T 3 : WR = 1,, M(AB) AD 7 -AD 0, (SP) (SP) -1 Machine Cycle- 3: MWRMC: Status signals IO/M=0, S 1 =1, S 0 =1 T 2 : WR = 0, AD 7 -AD 0 (PCL) T 3 : WR = 1,, M(AB) AD 7 -AD 0, Machine Cycle- 4 or (Machine Cycle-1 of next instruction): T 1 : A 15 -A 8 (W), AD 7 -AD 0 (Z), ALE = T 2 : RD = 0, FEO [(PC) (W, Z) +1], AD 7 -AD 0 M(AB) Thus, it requires three machine cycles OFMC, & two MWRMC and total 12 states. Using a 2 MH 3 internal clock it requires 6μsec.

6 One important point while writing a subroutine program starting from the fixed location as calculated. Consider for example RST 5 & RST 6. The corresponding starting addresses of the subroutine are 0028 H for RST 5 & 0030 H for RST 6. There is RST 5.5 interrupt control signal input at pin no 9 of the μp. This also corresponding to an interrupt service routine starting from 002C H. These addresses are shown in fig C3 RST Y 1 Y 0 Y 3 Y 2 Y 1 Y 0 RST- 5 subroutine address 002A H Y 3 Y 2 002B H 00 No operation 002C H C3 RST D H Z 1 Z 0 Z 3 Z 2 Z 1 Z 0 RST 5 subroutine address 002E H Z 3 Z 2 002F H 00 No operation 0030 H C3 RST H X 1 X 0 X 3 X 2 X 1 X 0 RST- 6 subroutine address 0032 H X 3 Y H 00 The figure shows that there are only four bytes, possible for each subroutine link and it is difficult to write subroutine here. Therefore, an unconditional jump instruction if 3-byts is written here. First byte will be C3 H (jump Instruction) and 2 nd & 3 rd bytes will be the address of some memory locations which is the starting address of subroutine. This is known as subroutine link address 4 th byte is no operation.

7 8. PCHL: This is a single byte instruction. The operation code is N There is no variation of this instruction. The macro RTL implemented is, (PCL) (L) (PCH) (H) Or (PC) (H,L) The meaning of the instruction is Move the content of register (H) to the higher byte of program counter (PCH) and move the content of register (L) to lower byte of program counter (PCL). In this way the control is transferred to the memory location whose address is available in (H, L) register pair. This is called register indirect jump instruction. This is the only instruction available in 8085A which allows the user to obtain a jump address from a register pair. This instruction uses register addressing mode. Other jump instructions use immediate and register indirect addressing mode. This instruction is very useful in implementing select structure of the software program. The macro RTL flow is as shown below: Machine Cycle- 1: T 1 : A 15 -A 8 (PCH), AD 7 -AD 0 (PCL), ALE = T 2 : RD = 0, (PC) (PC) +1, AD 7 -AD 0 M(AB) T 3 : RD = 1,, (IR) AD 7 -AD 0 T 4 : μp decodes the opcode. PCHL = 1. T 5 : (PCL) (L) T 6 : (PCH) (H) Thus 6 states are required no flags affected.

Machine Cycle- 2 or (Machine Cycle-1 of next instruction):

Machine Cycle- 2 or (Machine Cycle-1 of next instruction): Lecture-29 17. CMA: (Complement accumulator) This is an ALP statement. The meaning of the instruction is Complement the content of accumulator bit by bit and store the result back into the accumulator.