83Plus:Ports:04

Synopsis

Port Number: 04h

Function: Interrupting Device Identification and Memory Map Control

This port serves two purposes. When read it indicates the device that triggered an interrupt. When written it sets the memory map mode and hardware timer speed. When an interrupt is triggered, it should be cleared by reseting the corresponding bit in port 3. Otherwise, it will continously call the interrupt code once interrupts are reenabled with ei.

Read Values

• Bits 0~2 and 4~7 are set according to which device triggered the running interrupt.
  • Bit 0: Set if pressing the ON Key triggered the interrupt.
  • Bit 1: Set if the first hardware timer triggered the interrupt.
  • Bit 2: Set if the second hardware timer triggered the interrupt.
  • Bit 4: Link activity generated an interrupt.
  • 83+SE / 84+ only: Bit 5: First crystal timer has expired.
  • 83+SE / 84+ only: Bit 6: Second crystal timer has expired.
  • 83+SE / 84+ only: Bit 7: Third crystal timer has expired.
• Bit 3 is reset if the ON key is being pressed, set otherwise.

Write Values

• Bit 0 reset to select memory map mode 0. In mode 0 the RAM and ROM is mapped to CPU memory as follows:
  • Address 0000h ~ 3FFFh: ROM Page 0
  • Address 4000h ~ 7FFFh: Memory Bank A (Page selected in Port 06h)
  • Address 8000h ~ BFFFh: Memory Bank B (Page selected in Port 07h)
  • 83+ Basic: Address C000h ~ FFFFh: RAM Page 0
  • Everything else: Address C000h ~ FFFFh: Page selected in Port 05h
• Bit 0 set to select memory map mode 1. In mode 1 the RAM and ROM is mapped to CPU memory as follows:
  • Address 0000h ~ 3FFFh: ROM Page 0
  • Address 4000h ~ 7FFFh: RAM Page 0
  • Address 8000h ~ BFFFh: Memory Bank A (Page selected in Port 06h)
  • Address C000h ~ FFFFh: Memory Bank B (Page selected in Port 07h)
• Bits 1 and 2 control the hardware timer frequency. Setting both 0 sets the timer to the fastest speed, and both 1 is the slowest speed. The normal speed is with both bits 1.

Frequency (Hz)

	first timer		second timer		both enabled
value	83+	83+SE 84+(SE)	83+	83+SE 84+(SE)	83+	83+SE 84+(SE)
00	560	512	1120	1024	1680	1536
01	248	227	497	455	746	682
10	170	156	344	315	517	473
11	118	108	236	216	353	323

• Bits 4~6 should be 1 on the 83+ and 83+SE, and 0 on the 84+ and 84+SE.

Comments

The calculator uses mode 0 for normal operation. If you change the memory map mode be sure to change it back before returning control. Also, do not switch from mode 0 to mode 1 inside the 4000h ~ 7FFFh range, as it will basically crash the calculator because RAM Page 0 will take over that section and RAM Page 0 cannot execute code.

Interrupt timers seem to be independant of the CPU speed, so setting an SE calculator to 15mhz will not increase the timer frequency.

Example

This example shows successful use of this port, switching to mode 1 and back. Mode 1 is the only way to execute code beyond address C000h on the 83+ Basic. (Assumes code is running from 4000h (i.e. an application))

 in a, (7) ;Save current Bank B page because we'll trash it.
 push af
 in a,(6) ;Put this app's page into 8000
 out (7), a
 jp $ + 4003h ;This will actually jump to the next statement, but in Bank B.
 ld a, 77h ;Select mode 1 and keep the timer speed at normal...
 out (4), a ;Now we're in mode 1.
 ;We're still at 8000h here but we are back in Bank A.
 ;Now to go back.
 ld a, 76h ;I could dec a for this example, but...
 out (4), a ;Back in mode 0 and bank B.
 jp $ - 3FFDh ;Jump back to bank A.
 pop af
 out (7), a ;Restore bank B.

Credits and Contributions

• Dan Englender: Originally documented memory map modes here
• Michael Vincent: For his docs here, which helped me figure out the interrupt device bits.
• James Montelongo: For his docs here on the hardware timer.