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An addressing mode specifies how to calculate the effective memory address of an operand by using information held in registers and/or constants contained within a machine instruction or elsewhere. In computer programming, addressing modes are primarily of interest to those who write in assembly languages and to compiler writers.
The cycle stealing mode is used in systems in which the CPU should not be disabled for the length of time needed for burst transfer modes. In the cycle stealing mode, the DMA controller obtains access to the system bus the same way as in burst mode, using BR ( Bus Request ) and BG ( Bus Grant ) signals, which are the two signals controlling the ...
However, more typical, or frequent, "CISC" instructions merely combine a basic ALU operation, such as "add", with the access of one or more operands in memory (using addressing modes such as direct, indirect, indexed, etc.). Certain architectures may allow two or three operands (including the result) directly in memory or may be able to perform ...
Format 4: Only valid on SIC/XE machines, consists of the same elements as format 3, but instead of a 12-bit displacement, stores a 20-bit address. Both format 3 and format 4 have six-bit flag values in them, consisting of the following flag bits: n: Indirect addressing flag; i: Immediate addressing flag; x: Indexed addressing flag
(Rare) models with >64 KiB of RAM address space extend the 16-bit RAM addressing limits with RAMPX, RAMPY, RAMPZ and RAMPD registers. These provide additional high bits for addressing modes which use the X, Y, or Z register pairs, respectively, or the direct addressing instructions LDS/STS. Unlike ROM access, there are no distinct "extended ...
Architectures typically allow instructions to include some combination of operand addressing modes: Direct The instruction specifies a complete address Immediate The instruction specifies a value rather than an address Indexed The instruction specifies a register to use as an index. In some architecture the index is scaled by the operand length ...
As an example, the 6800 includes an 8-bit direct addressing mode used to make code smaller and faster; instead of a memory address having 16 bits and thus requiring two bytes to specify, commonly used variables could be located within a 256-byte window (called the "direct page" on the 6800 or "zero page" on the 6502 because of its location at ...
In contrast to the PDP-11's 3-bit fields, the VAX-11's 4-bit sub-bytes resulted in 16 addressing modes (0–15). However, addressing modes 0–3 were "short immediate" for immediate data of 6 bits or less (the 2 low-order bits of the addressing mode being the 2 high-order bits of the immediate data, when prepended to the remaining 4 bits in ...