Improve and slightly repair clock handling

readme.md

@@ -23,8 +23,9 @@ This is intended to simulate an LED-blinky by _toggling_ between _ON_ and _OFF_.
 
 The blinky _ON_/_OFF_ timebase is derived from a $1s$ tick,
 resulting in a toggle-frequency of ${\sim}{\frac{1}{2}}Hz$.
-The underlying timer resides on `port 0x45`. It is only available on the TI-84,
-not the TI-83.
+The underlying 32-bit timer resides on ports `0x41`-`0x44`,
+where the lower byte at port `0x41` is used.
+This timer is only available on the TI-84, not the TI-83.
 
 The blinky _ON_/_OFF_ text messages are printed
 at row $2$, column $7$.
@@ -104,15 +105,16 @@ the [license](./LICENSE) details.
 Some of this software has been reverse engineered
 from other projects or terse technical notes.
 In addition, the software writes to calculator `ports`,
-such as `port 0x41`.
+such as ports `0x40` and `0x41`.
 
 Personally. I do not consider this to be a $100\\%$ sound basis
 for project reliability and/or stability.
 
 Nonetheless, empirical evidence shows a working,
 and fully/properly initialized C-language application.
-It provides a basis that seems to adhere to the established, common
-rules of modern C. Larger projects could be built or modelled from this.
+It provides a basis that seems to adhere to the established,
+common rules of modern C. Larger projects could be built or modelled
+from this.
 
 Origins
   - This project has been inspired by and influenced by numerous previous works, including (but not limited to) the [azertyfun/LibTI](https://github.com/azertyfun/LibTI) project.

src/startup/clock.c

@@ -14,14 +14,6 @@ static void    clock_seconds_do_start   (void) ATTRIBUTE_NAKED;
 static void    clock_seconds_do_stop    (void) ATTRIBUTE_NAKED;
 static uint8_t clock_seconds_do_get     (void) ATTRIBUTE_NAKED;
 
-void _clock(void) ATTRIBUTE_NAKED
-{
-  // Mysterious _clock() method.
-
-  __asm__("ld a, #0x2\n");
-  __asm__("ret\n");
-}
-
 void clock_seconds_start(void)
 {
   const uint8_t clock_seconds_run_bit_value = (uint8_t) (clock_seconds_get_port40() & (uint8_t) UINT8_C(1));
@@ -59,23 +51,26 @@ static void clock_seconds_do_start(void) ATTRIBUTE_NAKED
 {
   // Reset the lower byte of the 32-bit clock.
   __asm__("xor a\n");
-  __asm__("out (0x44), a\n");
+  __asm__("out (0x41), a\n");
 
   // Ensure that the set command bit is off since its transition
   // to high will actually set the clock.
-  __asm__("ld a, #0x1\n");
+  __asm__("in a, (0x40)\n");
+  __asm__("and a, #0xFC\n");
+  __asm__("or a, #0x1\n");
   __asm__("out (0x40), a\n");
 
   // Set and start the clock.
-  __asm__("ld a, #0x3\n");
+  __asm__("or a, #0x3\n");
   __asm__("out (0x40), a\n");
   __asm__("ret\n");
 }
 
 static void clock_seconds_do_stop(void) ATTRIBUTE_NAKED
 {
   // Stop the clock.
-  __asm__("ld a, #0x0\n");
+  __asm__("in a, (0x40)\n");
+  __asm__("and a, #0xFE\n");
   __asm__("out (0x40), a\n");
   __asm__("ret\n");
 }