Difference between revisions of "83:Ports:04"

Revision as of 01:12, 3 April 2005

Synopsis

Port Number: 04h

Function: Timer Interrupt Frequency and Memory Map Control

This port controls the frequency of the hardware timer and the memory map mode.

Read Values

This is a mirror of the link port.

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: Swappable (Page selected in 83:Ports:00/83:Ports:02)
  • Address 8000h ~ BFFFh: RAM Page 1
  • Address C000h ~ FFFFh: RAM Page 0
• 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: Swappable (Page selected in 83:Ports:00/83:Ports:02)
  • Address C000h ~ FFFFh: RAM Page 1
• 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.
• Bit 3: Unused? Always 0.
• Bit 4: Timer frequency multiplier; when this bit is set, all the times are multiplied by 90% (i. e. it's 11.1% faster) compared to when it's reset.
• Bit 5-7: Unknown, but definitely used.

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.

To calculate resulting timer frequency you can use the following formula:

freq = 1800 * ((out_value & 0x10) ? 1.0 : 0.9) / (3 + ((out_value & 0x06) << 1)) / 6000000