Question

Compute the effective address and the content of ACC (accumulator) for a load instruction of a 1-address machine for each type of addressing modes using the following assumptions The load instruction is of length 4 bytes, the first byte is for op ode and mode and the other two bytes contain the value 90 for an address or an immediate value ? The load instruction is stored in locations 12-15 The register (say R1)contains the value 800; The location 800 contains the value 700; The location 90 contains the value 150; . The location 150 contains the value 77; . The location 102 contains the value 11; The location 106 contains the value 200; The location 890 contains the value 76 Direct Immediate Indirect Relative Index Register Register Indirect

Answer 1

Table showing the value of effective address and ACC values

Addressing Mode	Effective Address	Value of ACC(Accumulator)	Comment
Direct	90	150	In Direct Addressing mode, effective address is calculated using value stored in load instruction. Here, the value in instruction is 90 which is the effective address. Thus, value in ACC is 150.
Immediate	14	90	The value of Program Counter (current instruction pointer) is 12. Address is stored at 14 (given in the question) which is the effective address. So, value loaded in ACC is 90.
Indirect	150	77	For Indirect Addressing mode, we first go to address location 90. The effective address will be the value stored at 90 which is 150 and value at 150 is loaded in ACC. Thus, 77 is loaded into ACC.
Relative	16+90=106	200	Current Value of Program Counter (PC) is 12. Value of PC for next instruction is 16. So for relative addressing we add 90 to 16 (=106) to get the effective address for the load instruction i.e. the position which is 90 ahead of next instruction address value which is further used as effective address. Value in ACC will be 200.
Index	800+90=890	96	Using R1 as index register, in index addressing mode, effective address is the value of sum of index register and address in current instruction i.e. 800+90 = 890. Value in ACC is 96 i.e. value stored at 890.
Register	address of R1	800	Effective address in Register Addressing mode is the address of register itself. In some texts, it is also shown as none. Value in ACC is the value in the register R1 i.e. 800.
Register Indirect	800	700	In Register Indirect Mode, the value in the register is used as the effective address i.e. 800. The value in ACC will be value at location 800 which is 700.