5 Dec
2024
5 Dec
'24
2:21 a.m.
The 6809 has an 8 bit Direct Page register. This allows short memory
reference instructions on any 256 byte area in memory.
This would save both program bytes and CPU cycles.
On 12/4/2024 5:08 PM, Jon Elson via cctalk wrote:
On 12/4/24 14:19, Mike Katz via cctalk wrote:
I'm sorry but you don't need segment
registers for position
independent code at all.
Address all data off of the index registers or off the PC and all
branches are relative.
The PDP-11 had short relative jumps, etc. that were +/- 128 words, so
subroutines with small loops, etc. could run anywhere in memory
without rerunning the compiler/assembler.
The IBM 360 had a base register that was used in all memory
references, so the entire object deck (program section, data section,
constants) could be loaded anywhere in memory, then the base register
was set to the beginning of the partition, and it was all
relocatable. However, if the code stored a physical address at any
time (really common in 360 code) then the program could NOT be moved
after it started running.
Jon