83:Ports:04

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 controls the memory mapping mode. The default value for this bit is 0.
• 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 default 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

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

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