View unanswered posts | View active topics It is currently Thu Mar 28, 2024 2:50 pm



Reply to topic  [ 1 post ] 
 8085 versus 8085A 
Author Message

Joined: Sat Jun 07, 2014 12:57 am
Posts: 13
Hi,

The original 8085 was quickly replaced by the 8085A.

Among the differences are....

- The ALE output on the 8085 is tri-state
- The first S cycle (S0 and S1 output status pins) after a Reset on an 8085 will occur before the add lines are valid.
- An extra instruction fetch cycle occurs after the memory access is completed in an 8085, but makes no diff to timing or external devices
- During an instruction execute Bus idle machine cycle, the 8085A does not generate a hi pulse on ALE, where the 8085 pulses ALE on the T1 of every machine cycle.
- The 8085 puts ALE in tri-state when the CPU is in the Hold state, but the 8085A keeps ALE lo during the Hold state
- In Halt state the ALE is floated by the 8085 and kept lo by the 8085A
- The 8085A samples INTR during the next-to-last clock period of each instruction's execution. The 8085 samples INTR one clock period later.
- In the 8085 the INTA output serves as a both an interrupt ack and a read signal. This can cause timing issues with the 8085 during an interrupt, and you need to disable all IO device selections except the device being serviced to avoid any timing issues.
- The above issue may require wait states to be inserted in an 8085 system during interrupt acknowledgment.
- During multibyte instruction fetch during an interrupt, the 8085 pulses INTA rather than RD, on the 2nd & 3rd machine cycles
- The 8085A preserves the state of the interrupt enable flag after a TRAP
- The 1st RIM instruction after a TRAP on the 8085A will show what the interrupt was just prior to the TRAP

So, there are numerous differences which make the 8085 more problematic to timing issues around interrupts. Both the 8085 and 8085A run the same instruction sets and accept the same memory and peripheral devices.

What I don't know is how many, and for how long, the 8085 was produced before being replaced by the 8085A. I know it was replaced very early on, but that's about all.

I ponder this because I have an old SDK85 system that has an 8085 CPU. It's a plastic unit but it doesn't look like the "A" has rubbed away or anything like that. So I am wondering if I got one of these early run 8085s or just your usual 8085A but the markings have perished. I'll take a pic so you can see for yourselves.


Wed Dec 24, 2014 12:15 am
Profile
Display posts from previous:  Sort by  
Reply to topic   [ 1 post ] 

Who is online

Users browsing this forum: No registered users and 5 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group
Designed by ST Software