Z80:Ports:Port04

Basic Info

Port Address: 04

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 resetting the corresponding bit in port03. Otherwise, it will continuously call the interrupt code once interrupts are reenabled with EI.

Writing to Port

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 port06)
• Address 8000h ~ BFFFh: Memory Bank B (Page selected in port07)
• 83+ Basic: Address C000h ~ FFFFh: RAM Page 0
• Everything else: Address C000h ~ FFFFh: Page selected in port05

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: Memory Bank A, even page (value of port06 ANDed with FEh)
• 83+ Basic: Address 8000h ~ BFFFh: Memory Bank A (Page selected in port06)
• Everything else: Address 8000h ~ BFFFh: Memory Bank A, odd page (value of port06 ORed with 1)
• Address C000h ~ FFFFh: Memory Bank B (Page selected in port07)

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.

Reading from Port

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.

= 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	227.55	497	455.11	746	682.66
10	170	146.29	344	292.57	517	438.86
11	118	107.79	236	215.58	353	323.37

Bits 3~5: Unused. Bits 6 and 7:

• 83+ Basic: No function
• Everything else: These two bits are used to determine the battery state. They act as a 2-bit number and the higher it is, the higher the voltage cutoff for bit 0 of port02. Unless you are trying to determine battery state, these two bits should be zero.
  • TI-83+SE: Only has one bit, bit 7; the second bit was added for the 84-series.
  • TI-84+/SE: Numbers 00 and 10 are not currently used. 01 measures if the batteries are removed (0v). 11 measures if the main batteries are present but "low".
  • I-84+CSE: Based on the OS battery check routine, these two bits are in reverse order; that is, bit 6 is more significant than bit 7. (This probably also applies to the B&W models.) The routine always delays between changing these bits and checking bit 0 of port02. Also, for each check value, it writes 06 (i.e. resets the port) and then writes the value it intends to write. It writes, in order, 06, 86, 46, C6. If bit 0 of port02 is reset after any of those writes, it returns 0 through 3, respectively; if bit 0 of port02 remains set after all of those writes, it returns 4. Bit 7 of port3A is also related; that bit is normally kept reset, but is set for the duration of the test routine.

Comments

The calculator uses memory mapping 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 unless you've changed the RAM Execution Limits with port25 and port26. This is because switching modes in this manner will start executing on RAM page 0, which will crash under normal conditions. Interrupt timers may not be independent of the CPU speed for the 83+. However on the 83+SE, 84+ and 84+SE the standard timers are based on the crystal timer. Timer1 fires at 32768/(64+(80*i))hz, where i is bit 1 and 2 set in port04, making it 0<=i<=3.

Example Uses

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, unless you use a combination of ports port05 and port16 to enable execution on RAM page 00. (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.

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