Editing Addressing mode (section)

===Memory-mapped registers===
On some computers, there were addresses that referred to registers rather than to primary storage, or to primary memory used to implement those registers. Although on some early computers there were register addresses at the high end of the address range, e.g., [[IBM 650]],<ref>{{cite manual
 | title       = 650 magnetic drum data-processing machine - manual of operation
 | id          = 22-6060-2
 | date        = June 1955
 | page        = [http://www.bitsavers.org/pdf/ibm/650/22-6060-2_650_OperMan.pdf#page=8 9] 
 | section     = Storage
 | section-url = http://www.bitsavers.org/pdf/ibm/650/22-6060-2_650_OperMan.pdf#page=7
 | url         = http://www.bitsavers.org/pdf/ibm/650/22-6060-2_650_OperMan.pdf
 | accessdate  = March 14, 2022
 }}
</ref>{{efn|Condensor storage units for 650:
:8000 Console switches
:8001 Distributor
:8002 Lower accumulator
:8003 Upper accumulator
 }} [[IBM 7070]],<ref>{{cite manual
 | title       = Reference Manual - IBM 7070 Data Processing System
 | id          = A22-7003-0
 | date        = January 1960
 | page        = 252
 | section     = Core Storage and Register Addresses
 | section-url = http://www.bitsavers.org/pdf/ibm/7070/A22-7003-01_7070_Reference_Jan60.pdf#page=253
 | url         = http://www.bitsavers.org/pdf/ibm/7070/A22-7003-01_7070_Reference_Jan60.pdf
 | accessdate  = March 14, 2022
 }}
</ref>{{efn|For 5K or 10K 7070
:00xx Index register xx
:9991 Accumulator 1
:9992 Accumulator 2
:9993 Accumulator 3
:9995 Program register{{efn|Only valid from console|name=console}}
:9999 Instruction counter{{efn|name=console}}
 }} the trend has been to use only register address at the low end and to use only the first 8 or 16 words of memory (e.g. [[ICT 1900 series|ICL 1900]], DEC PDP-6/PDP-10). This meant that there was no need for a separate "add register to register" instruction – one could just use the "add memory to register" instruction.

In the case of early models of the PDP-10, which did not have any cache memory, if the "fast registers" option, which provided faster circuits to store the registers but still allowed them to be addressed as if they were in memory, was installed, a tight inner loop loaded into the first few words of memory ran much faster than it would have in magnetic core memory.

Later models of the DEC [[PDP-11]] series mapped the registers onto addresses in the input/output area, but this was primarily intended to allow remote diagnostics. Confusingly, the 16-bit registers were mapped onto consecutive 8-bit byte addresses.